[PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[Rafael Aquini]

read_swap_cache_async() can race against get_swap_page(), and stumble across
a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
swapcache yet. This transient swap_map state is expected to be transitory,
but the actual placement of discard at scan_swap_map() inserts a wait for
I/O completion thus making the thread at read_swap_cache_async() to loop
around its -EEXIST case, while the other end at get_swap_page()
is scheduled away at scan_swap_map(). This can leave the system deadlocked
if the I/O completion happens to be waiting on the CPU workqueue where
read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.

This patch introduces a cond_resched() call to make the aforementioned
read_swap_cache_async() busy loop condition to bail out when necessary,
thus avoiding the subtle race window.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
 mm/swap_state.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index b3d40dc..9ad9e3b 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 * Swap entry may have been freed since our caller observed it.
 		 */
 		err = swapcache_prepare(entry);
-		if (err == -EEXIST) {	/* seems racy */
+		if (err == -EEXIST) {
 			radix_tree_preload_end();
+			/*
+			 * We might race against get_swap_page() and stumble
+			 * across a SWAP_HAS_CACHE swap_map entry whose page
+			 * has not been brought into the swapcache yet, while
+			 * the other end is scheduled away waiting on discard
+			 * I/O completion.
+			 * In order to avoid turning this transitory state
+			 * into a permanent loop around this -EEXIST case,
+			 * lets just conditionally invoke the scheduler,
+			 * if there are some more important tasks to run.
+			 */
+			cond_resched();
 			continue;
 		}
 		if (err) {		/* swp entry is obsolete ? */
-- 
1.8.1.4

[PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[Rafael Aquini]

read_swap_cache_async() can race against get_swap_page(), and stumble across
a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
swapcache yet. This transient swap_map state is expected to be transitory,
but the actual placement of discard at scan_swap_map() inserts a wait for
I/O completion thus making the thread at read_swap_cache_async() to loop
around its -EEXIST case, while the other end at get_swap_page()
is scheduled away at scan_swap_map(). This can leave the system deadlocked
if the I/O completion happens to be waiting on the CPU workqueue where
read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.

This patch introduces a cond_resched() call to make the aforementioned
read_swap_cache_async() busy loop condition to bail out when necessary,
thus avoiding the subtle race window.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
 mm/swap_state.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index b3d40dc..9ad9e3b 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 * Swap entry may have been freed since our caller observed it.
 		 */
 		err = swapcache_prepare(entry);
-		if (err == -EEXIST) {	/* seems racy */
+		if (err == -EEXIST) {
 			radix_tree_preload_end();
+			/*
+			 * We might race against get_swap_page() and stumble
+			 * across a SWAP_HAS_CACHE swap_map entry whose page
+			 * has not been brought into the swapcache yet, while
+			 * the other end is scheduled away waiting on discard
+			 * I/O completion.
+			 * In order to avoid turning this transitory state
+			 * into a permanent loop around this -EEXIST case,
+			 * lets just conditionally invoke the scheduler,
+			 * if there are some more important tasks to run.
+			 */
+			cond_resched();
 			continue;
 		}
 		if (err) {		/* swp entry is obsolete ? */
-- 
1.8.1.4

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Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[Greg KH]

On Thu, May 30, 2013 at 03:05:00PM -0300, Rafael Aquini wrote:
> read_swap_cache_async() can race against get_swap_page(), and stumble across
> a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> swapcache yet. This transient swap_map state is expected to be transitory,
> but the actual placement of discard at scan_swap_map() inserts a wait for
> I/O completion thus making the thread at read_swap_cache_async() to loop
> around its -EEXIST case, while the other end at get_swap_page()
> is scheduled away at scan_swap_map(). This can leave the system deadlocked
> if the I/O completion happens to be waiting on the CPU workqueue where
> read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
> 
> This patch introduces a cond_resched() call to make the aforementioned
> read_swap_cache_async() busy loop condition to bail out when necessary,
> thus avoiding the subtle race window.
> 
> Signed-off-by: Rafael Aquini <aquini@redhat.com>
> ---
>  mm/swap_state.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)


<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read Documentation/stable_kernel_rules.txt
for how to do this properly.

</formletter>

Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[Greg KH]

On Thu, May 30, 2013 at 03:05:00PM -0300, Rafael Aquini wrote:
> read_swap_cache_async() can race against get_swap_page(), and stumble across
> a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> swapcache yet. This transient swap_map state is expected to be transitory,
> but the actual placement of discard at scan_swap_map() inserts a wait for
> I/O completion thus making the thread at read_swap_cache_async() to loop
> around its -EEXIST case, while the other end at get_swap_page()
> is scheduled away at scan_swap_map(). This can leave the system deadlocked
> if the I/O completion happens to be waiting on the CPU workqueue where
> read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
> 
> This patch introduces a cond_resched() call to make the aforementioned
> read_swap_cache_async() busy loop condition to bail out when necessary,
> thus avoiding the subtle race window.
> 
> Signed-off-by: Rafael Aquini <aquini@redhat.com>
> ---
>  mm/swap_state.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)


<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read Documentation/stable_kernel_rules.txt
for how to do this properly.

</formletter>

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Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[Johannes Weiner]

On Thu, May 30, 2013 at 03:05:00PM -0300, Rafael Aquini wrote:
> read_swap_cache_async() can race against get_swap_page(), and stumble across
> a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> swapcache yet. This transient swap_map state is expected to be transitory,
> but the actual placement of discard at scan_swap_map() inserts a wait for
> I/O completion thus making the thread at read_swap_cache_async() to loop
> around its -EEXIST case, while the other end at get_swap_page()
> is scheduled away at scan_swap_map(). This can leave the system deadlocked
> if the I/O completion happens to be waiting on the CPU workqueue where

waitqueue?

> read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
> 
> This patch introduces a cond_resched() call to make the aforementioned
> read_swap_cache_async() busy loop condition to bail out when necessary,
> thus avoiding the subtle race window.
> 
> Signed-off-by: Rafael Aquini <aquini@redhat.com>
> ---
>  mm/swap_state.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index b3d40dc..9ad9e3b 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  		 * Swap entry may have been freed since our caller observed it.
>  		 */
>  		err = swapcache_prepare(entry);
> -		if (err == -EEXIST) {	/* seems racy */
> +		if (err == -EEXIST) {
>  			radix_tree_preload_end();
> +			/*
> +			 * We might race against get_swap_page() and stumble
> +			 * across a SWAP_HAS_CACHE swap_map entry whose page
> +			 * has not been brought into the swapcache yet, while
> +			 * the other end is scheduled away waiting on discard
> +			 * I/O completion.
> +			 * In order to avoid turning this transitory state
> +			 * into a permanent loop around this -EEXIST case,
> +			 * lets just conditionally invoke the scheduler,
> +			 * if there are some more important tasks to run.
> +			 */
> +			cond_resched();

Might be worth mentioning the !CONFIG_PREEMPT deadlock scenario here,
especially since under CONFIG_PREEMPT the radix_tree_preload_end() is
already a scheduling point through the preempt_enable().

Other than that, the patch looks good to me!

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[Johannes Weiner]

On Thu, May 30, 2013 at 03:05:00PM -0300, Rafael Aquini wrote:
> read_swap_cache_async() can race against get_swap_page(), and stumble across
> a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> swapcache yet. This transient swap_map state is expected to be transitory,
> but the actual placement of discard at scan_swap_map() inserts a wait for
> I/O completion thus making the thread at read_swap_cache_async() to loop
> around its -EEXIST case, while the other end at get_swap_page()
> is scheduled away at scan_swap_map(). This can leave the system deadlocked
> if the I/O completion happens to be waiting on the CPU workqueue where

waitqueue?

> read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
> 
> This patch introduces a cond_resched() call to make the aforementioned
> read_swap_cache_async() busy loop condition to bail out when necessary,
> thus avoiding the subtle race window.
> 
> Signed-off-by: Rafael Aquini <aquini@redhat.com>
> ---
>  mm/swap_state.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index b3d40dc..9ad9e3b 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  		 * Swap entry may have been freed since our caller observed it.
>  		 */
>  		err = swapcache_prepare(entry);
> -		if (err == -EEXIST) {	/* seems racy */
> +		if (err == -EEXIST) {
>  			radix_tree_preload_end();
> +			/*
> +			 * We might race against get_swap_page() and stumble
> +			 * across a SWAP_HAS_CACHE swap_map entry whose page
> +			 * has not been brought into the swapcache yet, while
> +			 * the other end is scheduled away waiting on discard
> +			 * I/O completion.
> +			 * In order to avoid turning this transitory state
> +			 * into a permanent loop around this -EEXIST case,
> +			 * lets just conditionally invoke the scheduler,
> +			 * if there are some more important tasks to run.
> +			 */
> +			cond_resched();

Might be worth mentioning the !CONFIG_PREEMPT deadlock scenario here,
especially since under CONFIG_PREEMPT the radix_tree_preload_end() is
already a scheduling point through the preempt_enable().

Other than that, the patch looks good to me!

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[KOSAKI Motohiro]

On Thu, May 30, 2013 at 2:05 PM, Rafael Aquini <aquini@redhat.com> wrote:
> read_swap_cache_async() can race against get_swap_page(), and stumble across
> a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> swapcache yet. This transient swap_map state is expected to be transitory,
> but the actual placement of discard at scan_swap_map() inserts a wait for
> I/O completion thus making the thread at read_swap_cache_async() to loop
> around its -EEXIST case, while the other end at get_swap_page()
> is scheduled away at scan_swap_map(). This can leave the system deadlocked
> if the I/O completion happens to be waiting on the CPU workqueue where
> read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
>
> This patch introduces a cond_resched() call to make the aforementioned
> read_swap_cache_async() busy loop condition to bail out when necessary,
> thus avoiding the subtle race window.
>
> Signed-off-by: Rafael Aquini <aquini@redhat.com>

Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[KOSAKI Motohiro]

On Thu, May 30, 2013 at 2:05 PM, Rafael Aquini <aquini@redhat.com> wrote:
> read_swap_cache_async() can race against get_swap_page(), and stumble across
> a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> swapcache yet. This transient swap_map state is expected to be transitory,
> but the actual placement of discard at scan_swap_map() inserts a wait for
> I/O completion thus making the thread at read_swap_cache_async() to loop
> around its -EEXIST case, while the other end at get_swap_page()
> is scheduled away at scan_swap_map(). This can leave the system deadlocked
> if the I/O completion happens to be waiting on the CPU workqueue where
> read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
>
> This patch introduces a cond_resched() call to make the aforementioned
> read_swap_cache_async() busy loop condition to bail out when necessary,
> thus avoiding the subtle race window.
>
> Signed-off-by: Rafael Aquini <aquini@redhat.com>

Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

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Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[Rafael Aquini]

On Thu, May 30, 2013 at 03:55:39PM -0400, Johannes Weiner wrote:
> On Thu, May 30, 2013 at 03:05:00PM -0300, Rafael Aquini wrote:
> > read_swap_cache_async() can race against get_swap_page(), and stumble across
> > a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> > swapcache yet. This transient swap_map state is expected to be transitory,
> > but the actual placement of discard at scan_swap_map() inserts a wait for
> > I/O completion thus making the thread at read_swap_cache_async() to loop
> > around its -EEXIST case, while the other end at get_swap_page()
> > is scheduled away at scan_swap_map(). This can leave the system deadlocked
> > if the I/O completion happens to be waiting on the CPU workqueue where
> 
> waitqueue?
>

Ugh! I will repost this to fix it and the "compeletion" typo at subject...


> > read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
> > 
> > This patch introduces a cond_resched() call to make the aforementioned
> > read_swap_cache_async() busy loop condition to bail out when necessary,
> > thus avoiding the subtle race window.
> > 
> > Signed-off-by: Rafael Aquini <aquini@redhat.com>
> > ---
> >  mm/swap_state.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index b3d40dc..9ad9e3b 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> >  		 * Swap entry may have been freed since our caller observed it.
> >  		 */
> >  		err = swapcache_prepare(entry);
> > -		if (err == -EEXIST) {	/* seems racy */
> > +		if (err == -EEXIST) {
> >  			radix_tree_preload_end();
> > +			/*
> > +			 * We might race against get_swap_page() and stumble
> > +			 * across a SWAP_HAS_CACHE swap_map entry whose page
> > +			 * has not been brought into the swapcache yet, while
> > +			 * the other end is scheduled away waiting on discard
> > +			 * I/O completion.
> > +			 * In order to avoid turning this transitory state
> > +			 * into a permanent loop around this -EEXIST case,
> > +			 * lets just conditionally invoke the scheduler,
> > +			 * if there are some more important tasks to run.
> > +			 */
> > +			cond_resched();
> 
> Might be worth mentioning the !CONFIG_PREEMPT deadlock scenario here,
> especially since under CONFIG_PREEMPT the radix_tree_preload_end() is
> already a scheduling point through the preempt_enable().
>

Nice suggestion, will do it. Thanks for reviewing this patch!


> Other than that, the patch looks good to me!
> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[Rafael Aquini]

On Thu, May 30, 2013 at 03:55:39PM -0400, Johannes Weiner wrote:
> On Thu, May 30, 2013 at 03:05:00PM -0300, Rafael Aquini wrote:
> > read_swap_cache_async() can race against get_swap_page(), and stumble across
> > a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> > swapcache yet. This transient swap_map state is expected to be transitory,
> > but the actual placement of discard at scan_swap_map() inserts a wait for
> > I/O completion thus making the thread at read_swap_cache_async() to loop
> > around its -EEXIST case, while the other end at get_swap_page()
> > is scheduled away at scan_swap_map(). This can leave the system deadlocked
> > if the I/O completion happens to be waiting on the CPU workqueue where
> 
> waitqueue?
>

Ugh! I will repost this to fix it and the "compeletion" typo at subject...


> > read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
> > 
> > This patch introduces a cond_resched() call to make the aforementioned
> > read_swap_cache_async() busy loop condition to bail out when necessary,
> > thus avoiding the subtle race window.
> > 
> > Signed-off-by: Rafael Aquini <aquini@redhat.com>
> > ---
> >  mm/swap_state.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index b3d40dc..9ad9e3b 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> >  		 * Swap entry may have been freed since our caller observed it.
> >  		 */
> >  		err = swapcache_prepare(entry);
> > -		if (err == -EEXIST) {	/* seems racy */
> > +		if (err == -EEXIST) {
> >  			radix_tree_preload_end();
> > +			/*
> > +			 * We might race against get_swap_page() and stumble
> > +			 * across a SWAP_HAS_CACHE swap_map entry whose page
> > +			 * has not been brought into the swapcache yet, while
> > +			 * the other end is scheduled away waiting on discard
> > +			 * I/O completion.
> > +			 * In order to avoid turning this transitory state
> > +			 * into a permanent loop around this -EEXIST case,
> > +			 * lets just conditionally invoke the scheduler,
> > +			 * if there are some more important tasks to run.
> > +			 */
> > +			cond_resched();
> 
> Might be worth mentioning the !CONFIG_PREEMPT deadlock scenario here,
> especially since under CONFIG_PREEMPT the radix_tree_preload_end() is
> already a scheduling point through the preempt_enable().
>

Nice suggestion, will do it. Thanks for reviewing this patch!


> Other than that, the patch looks good to me!
> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[Hugh Dickins]

On Thu, 30 May 2013, Rafael Aquini wrote:

> read_swap_cache_async() can race against get_swap_page(), and stumble across
> a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> swapcache yet. This transient swap_map state is expected to be transitory,
> but the actual placement of discard at scan_swap_map() inserts a wait for
> I/O completion thus making the thread at read_swap_cache_async() to loop
> around its -EEXIST case, while the other end at get_swap_page()
> is scheduled away at scan_swap_map(). This can leave the system deadlocked
> if the I/O completion happens to be waiting on the CPU workqueue where
> read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
> 
> This patch introduces a cond_resched() call to make the aforementioned
> read_swap_cache_async() busy loop condition to bail out when necessary,
> thus avoiding the subtle race window.

Yes, I never realized this at the time I inserted discard there.
As you know, Shaohua has a better swap discard implementation, which
avoids the problem by using SWAP_MAP_BAD, but this cond_resched() is
a good simple workaround for now - thanks.

> 
> Signed-off-by: Rafael Aquini <aquini@redhat.com>

Acked-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org

> ---
>  mm/swap_state.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index b3d40dc..9ad9e3b 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  		 * Swap entry may have been freed since our caller observed it.
>  		 */
>  		err = swapcache_prepare(entry);
> -		if (err == -EEXIST) {	/* seems racy */
> +		if (err == -EEXIST) {
>  			radix_tree_preload_end();
> +			/*
> +			 * We might race against get_swap_page() and stumble
> +			 * across a SWAP_HAS_CACHE swap_map entry whose page
> +			 * has not been brought into the swapcache yet, while
> +			 * the other end is scheduled away waiting on discard
> +			 * I/O completion.
> +			 * In order to avoid turning this transitory state
> +			 * into a permanent loop around this -EEXIST case,
> +			 * lets just conditionally invoke the scheduler,
> +			 * if there are some more important tasks to run.
> +			 */
> +			cond_resched();
>  			continue;
>  		}
>  		if (err) {		/* swp entry is obsolete ? */
> -- 
> 1.8.1.4

Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion

[Hugh Dickins]

On Thu, 30 May 2013, Rafael Aquini wrote:

> read_swap_cache_async() can race against get_swap_page(), and stumble across
> a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> swapcache yet. This transient swap_map state is expected to be transitory,
> but the actual placement of discard at scan_swap_map() inserts a wait for
> I/O completion thus making the thread at read_swap_cache_async() to loop
> around its -EEXIST case, while the other end at get_swap_page()
> is scheduled away at scan_swap_map(). This can leave the system deadlocked
> if the I/O completion happens to be waiting on the CPU workqueue where
> read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
> 
> This patch introduces a cond_resched() call to make the aforementioned
> read_swap_cache_async() busy loop condition to bail out when necessary,
> thus avoiding the subtle race window.

Yes, I never realized this at the time I inserted discard there.
As you know, Shaohua has a better swap discard implementation, which
avoids the problem by using SWAP_MAP_BAD, but this cond_resched() is
a good simple workaround for now - thanks.

> 
> Signed-off-by: Rafael Aquini <aquini@redhat.com>

Acked-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org

> ---
>  mm/swap_state.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index b3d40dc..9ad9e3b 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  		 * Swap entry may have been freed since our caller observed it.
>  		 */
>  		err = swapcache_prepare(entry);
> -		if (err == -EEXIST) {	/* seems racy */
> +		if (err == -EEXIST) {
>  			radix_tree_preload_end();
> +			/*
> +			 * We might race against get_swap_page() and stumble
> +			 * across a SWAP_HAS_CACHE swap_map entry whose page
> +			 * has not been brought into the swapcache yet, while
> +			 * the other end is scheduled away waiting on discard
> +			 * I/O completion.
> +			 * In order to avoid turning this transitory state
> +			 * into a permanent loop around this -EEXIST case,
> +			 * lets just conditionally invoke the scheduler,
> +			 * if there are some more important tasks to run.
> +			 */
> +			cond_resched();
>  			continue;
>  		}
>  		if (err) {		/* swp entry is obsolete ? */
> -- 
> 1.8.1.4

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[PATCH v2] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O completion

[Rafael Aquini]

read_swap_cache_async() can race against get_swap_page(), and stumble across
a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
swapcache yet. This transient swap_map state is expected to be transitory,
but the actual placement of discard at scan_swap_map() inserts a wait for
I/O completion thus making the thread at read_swap_cache_async() to loop
around its -EEXIST case, while the other end at get_swap_page()
is scheduled away at scan_swap_map(). This can leave the system deadlocked
if the I/O completion happens to be waiting on the CPU waitqueue where
read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.

This patch introduces a cond_resched() call to make the aforementioned
read_swap_cache_async() busy loop condition to bail out when necessary,
thus avoiding the subtle race window.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org
---
Changelog:
* fixed typos in commit message;			(hannes)
* enhanced the commentary on the deadlock scenario;	(hannes)
* include the received ACKs and Cc: stable

 mm/swap_state.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index b3d40dc..f24ab0d 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -336,8 +336,24 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 * Swap entry may have been freed since our caller observed it.
 		 */
 		err = swapcache_prepare(entry);
-		if (err == -EEXIST) {	/* seems racy */
+		if (err == -EEXIST) {
 			radix_tree_preload_end();
+			/*
+			 * We might race against get_swap_page() and stumble
+			 * across a SWAP_HAS_CACHE swap_map entry whose page
+			 * has not been brought into the swapcache yet, while
+			 * the other end is scheduled away waiting on discard
+			 * I/O completion at scan_swap_map().
+			 *
+			 * In order to avoid turning this transitory state
+			 * into a permanent loop around this -EEXIST case
+			 * if !CONFIG_PREEMPT and the I/O completion happens
+			 * to be waiting on the CPU waitqueue where we are now
+			 * busy looping, we just conditionally invoke the
+			 * scheduler here, if there are some more important
+			 * tasks to run.
+			 */
+			cond_resched();
 			continue;
 		}
 		if (err) {		/* swp entry is obsolete ? */
-- 
1.8.1.4

[PATCH v2] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O completion

[Rafael Aquini]

read_swap_cache_async() can race against get_swap_page(), and stumble across
a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
swapcache yet. This transient swap_map state is expected to be transitory,
but the actual placement of discard at scan_swap_map() inserts a wait for
I/O completion thus making the thread at read_swap_cache_async() to loop
around its -EEXIST case, while the other end at get_swap_page()
is scheduled away at scan_swap_map(). This can leave the system deadlocked
if the I/O completion happens to be waiting on the CPU waitqueue where
read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.

This patch introduces a cond_resched() call to make the aforementioned
read_swap_cache_async() busy loop condition to bail out when necessary,
thus avoiding the subtle race window.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org
---
Changelog:
* fixed typos in commit message;			(hannes)
* enhanced the commentary on the deadlock scenario;	(hannes)
* include the received ACKs and Cc: stable

 mm/swap_state.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index b3d40dc..f24ab0d 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -336,8 +336,24 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 * Swap entry may have been freed since our caller observed it.
 		 */
 		err = swapcache_prepare(entry);
-		if (err == -EEXIST) {	/* seems racy */
+		if (err == -EEXIST) {
 			radix_tree_preload_end();
+			/*
+			 * We might race against get_swap_page() and stumble
+			 * across a SWAP_HAS_CACHE swap_map entry whose page
+			 * has not been brought into the swapcache yet, while
+			 * the other end is scheduled away waiting on discard
+			 * I/O completion at scan_swap_map().
+			 *
+			 * In order to avoid turning this transitory state
+			 * into a permanent loop around this -EEXIST case
+			 * if !CONFIG_PREEMPT and the I/O completion happens
+			 * to be waiting on the CPU waitqueue where we are now
+			 * busy looping, we just conditionally invoke the
+			 * scheduler here, if there are some more important
+			 * tasks to run.
+			 */
+			cond_resched();
 			continue;
 		}
 		if (err) {		/* swp entry is obsolete ? */
-- 
1.8.1.4

--
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