[PATCH] btrfs: Fix race in read_extent_buffer_pages()

[PATCH] btrfs: Fix race in read_extent_buffer_pages()

[Tavian Barnes]

To prevent concurrent reads for the same extent buffer,
read_extent_buffer_pages() performs these checks:

    /* (1) */
    if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
        return 0;

    /* (2) */
    if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags))
        goto done;

At this point, it seems safe to start the actual read operation. Once
that completes, end_bbio_meta_read() does

    /* (3) */
    set_extent_buffer_uptodate(eb);

    /* (4) */
    clear_bit(EXTENT_BUFFER_READING, &eb->bflags);

Normally, this is enough to ensure only one read happens, and all other
callers wait for it to finish before returning.  Unfortunately, there is
a racey interleaving:

    Thread A | Thread B | Thread C
    ---------+----------+---------
       (1)   |          |
             |    (1)   |
       (2)   |          |
       (3)   |          |
       (4)   |          |
             |    (2)   |
             |          |    (1)

When this happens, thread B kicks of an unnecessary read. Worse, thread
C will see UPTODATE set and return immediately, while the read from
thread B is still in progress.  This race could result in tree-checker
errors like this as the extent buffer is concurrently modified:

    BTRFS critical (device dm-0): corrupted node, root=256
    block=8550954455682405139 owner mismatch, have 11858205567642294356
    expect [256, 18446744073709551360]

Fix it by testing UPTODATE again after setting the READING bit, and if
it's been set, skip the unnecessary read.

Fixes: d7172f52e993 ("btrfs: use per-buffer locking for extent_buffer reading")
Link: https://lore.kernel.org/linux-btrfs/CAHk-=whNdMaN9ntZ47XRKP6DBes2E5w7fi-0U3H2+PS18p+Pzw@mail.gmail.com/
Link: https://lore.kernel.org/linux-btrfs/f51a6d5d7432455a6a858d51b49ecac183e0bbc9.1706312914.git.wqu@suse.com/
Link: https://lore.kernel.org/linux-btrfs/c7241ea4-fcc6-48d2-98c8-b5ea790d6c89@gmx.com/
Signed-off-by: Tavian Barnes <tavianator@tavianator.com>
---
 fs/btrfs/extent_io.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 7441245b1ceb..61594eaf1f89 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4333,6 +4333,19 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num,
 	if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags))
 		goto done;
 
+	/*
+	 * Between the initial test_bit(EXTENT_BUFFER_UPTODATE) and the above
+	 * test_and_set_bit(EXTENT_BUFFER_READING), someone else could have
+	 * started and finished reading the same eb.  In this case, UPTODATE
+	 * will now be set, and we shouldn't read it in again.
+	 */
+	if (unlikely(test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))) {
+		clear_bit(EXTENT_BUFFER_READING, &eb->bflags);
+		smp_mb__after_atomic();
+		wake_up_bit(&eb->bflags, EXTENT_BUFFER_READING);
+		return 0;
+	}
+
 	clear_bit(EXTENT_BUFFER_READ_ERR, &eb->bflags);
 	eb->read_mirror = 0;
 	check_buffer_tree_ref(eb);
-- 
2.44.0

Re: [PATCH] btrfs: Fix race in read_extent_buffer_pages()

[Qu Wenruo]

在 2024/3/16 11:44, Tavian Barnes 写道:
> To prevent concurrent reads for the same extent buffer,
> read_extent_buffer_pages() performs these checks:
> 
>      /* (1) */
>      if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
>          return 0;
> 
>      /* (2) */
>      if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags))
>          goto done;
> 
> At this point, it seems safe to start the actual read operation. Once
> that completes, end_bbio_meta_read() does
> 
>      /* (3) */
>      set_extent_buffer_uptodate(eb);
> 
>      /* (4) */
>      clear_bit(EXTENT_BUFFER_READING, &eb->bflags);
> 
> Normally, this is enough to ensure only one read happens, and all other
> callers wait for it to finish before returning.  Unfortunately, there is
> a racey interleaving:
> 
>      Thread A | Thread B | Thread C
>      ---------+----------+---------
>         (1)   |          |
>               |    (1)   |
>         (2)   |          |
>         (3)   |          |
>         (4)   |          |
>               |    (2)   |
>               |          |    (1)
> 
> When this happens, thread B kicks of an unnecessary read. Worse, thread
> C will see UPTODATE set and return immediately, while the read from
> thread B is still in progress.  This race could result in tree-checker
> errors like this as the extent buffer is concurrently modified:
> 
>      BTRFS critical (device dm-0): corrupted node, root=256
>      block=8550954455682405139 owner mismatch, have 11858205567642294356
>      expect [256, 18446744073709551360]
> 
> Fix it by testing UPTODATE again after setting the READING bit, and if
> it's been set, skip the unnecessary read.
> 
> Fixes: d7172f52e993 ("btrfs: use per-buffer locking for extent_buffer reading")
> Link: https://lore.kernel.org/linux-btrfs/CAHk-=whNdMaN9ntZ47XRKP6DBes2E5w7fi-0U3H2+PS18p+Pzw@mail.gmail.com/
> Link: https://lore.kernel.org/linux-btrfs/f51a6d5d7432455a6a858d51b49ecac183e0bbc9.1706312914.git.wqu@suse.com/
> Link: https://lore.kernel.org/linux-btrfs/c7241ea4-fcc6-48d2-98c8-b5ea790d6c89@gmx.com/
> Signed-off-by: Tavian Barnes <tavianator@tavianator.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu
> ---
>   fs/btrfs/extent_io.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 7441245b1ceb..61594eaf1f89 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4333,6 +4333,19 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num,
>   	if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags))
>   		goto done;
>   
> +	/*
> +	 * Between the initial test_bit(EXTENT_BUFFER_UPTODATE) and the above
> +	 * test_and_set_bit(EXTENT_BUFFER_READING), someone else could have
> +	 * started and finished reading the same eb.  In this case, UPTODATE
> +	 * will now be set, and we shouldn't read it in again.
> +	 */
> +	if (unlikely(test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))) {
> +		clear_bit(EXTENT_BUFFER_READING, &eb->bflags);
> +		smp_mb__after_atomic();
> +		wake_up_bit(&eb->bflags, EXTENT_BUFFER_READING);
> +		return 0;
> +	}
> +
>   	clear_bit(EXTENT_BUFFER_READ_ERR, &eb->bflags);
>   	eb->read_mirror = 0;
>   	check_buffer_tree_ref(eb);

Re: [PATCH] btrfs: Fix race in read_extent_buffer_pages()

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH] btrfs: Fix race in read_extent_buffer_pages()

[David Sterba]

On Fri, Mar 15, 2024 at 09:14:29PM -0400, Tavian Barnes wrote:
> To prevent concurrent reads for the same extent buffer,
> read_extent_buffer_pages() performs these checks:
> 
>     /* (1) */
>     if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
>         return 0;
> 
>     /* (2) */
>     if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags))
>         goto done;
> 
> At this point, it seems safe to start the actual read operation. Once
> that completes, end_bbio_meta_read() does
> 
>     /* (3) */
>     set_extent_buffer_uptodate(eb);
> 
>     /* (4) */
>     clear_bit(EXTENT_BUFFER_READING, &eb->bflags);
> 
> Normally, this is enough to ensure only one read happens, and all other
> callers wait for it to finish before returning.  Unfortunately, there is
> a racey interleaving:
> 
>     Thread A | Thread B | Thread C
>     ---------+----------+---------
>        (1)   |          |
>              |    (1)   |
>        (2)   |          |
>        (3)   |          |
>        (4)   |          |
>              |    (2)   |
>              |          |    (1)
> 
> When this happens, thread B kicks of an unnecessary read. Worse, thread
> C will see UPTODATE set and return immediately, while the read from
> thread B is still in progress.  This race could result in tree-checker
> errors like this as the extent buffer is concurrently modified:
> 
>     BTRFS critical (device dm-0): corrupted node, root=256
>     block=8550954455682405139 owner mismatch, have 11858205567642294356
>     expect [256, 18446744073709551360]
> 
> Fix it by testing UPTODATE again after setting the READING bit, and if
> it's been set, skip the unnecessary read.
> 
> Fixes: d7172f52e993 ("btrfs: use per-buffer locking for extent_buffer reading")
> Link: https://lore.kernel.org/linux-btrfs/CAHk-=whNdMaN9ntZ47XRKP6DBes2E5w7fi-0U3H2+PS18p+Pzw@mail.gmail.com/
> Link: https://lore.kernel.org/linux-btrfs/f51a6d5d7432455a6a858d51b49ecac183e0bbc9.1706312914.git.wqu@suse.com/
> Link: https://lore.kernel.org/linux-btrfs/c7241ea4-fcc6-48d2-98c8-b5ea790d6c89@gmx.com/
> Signed-off-by: Tavian Barnes <tavianator@tavianator.com>

Thank you very much for taking the time to debug the issue and for the
fix. It is a rare occurrence that a tough bug is followed by a fix from
the same person (outside of the developer group) and is certainly
appreciated.

Re: [PATCH] btrfs: Fix race in read_extent_buffer_pages()

[Tavian Barnes]

On Fri, Mar 22, 2024 at 3:28 PM David Sterba <dsterba@suse.cz> wrote:
> On Fri, Mar 15, 2024 at 09:14:29PM -0400, Tavian Barnes wrote:
> > To prevent concurrent reads for the same extent buffer,
> > read_extent_buffer_pages() performs these checks:
> >
> >     /* (1) */
> >     if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
> >         return 0;
> >
> >     /* (2) */
> >     if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags))
> >         goto done;
> >
> > At this point, it seems safe to start the actual read operation. Once
> > that completes, end_bbio_meta_read() does
> >
> >     /* (3) */
> >     set_extent_buffer_uptodate(eb);
> >
> >     /* (4) */
> >     clear_bit(EXTENT_BUFFER_READING, &eb->bflags);
> >
> > Normally, this is enough to ensure only one read happens, and all other
> > callers wait for it to finish before returning.  Unfortunately, there is
> > a racey interleaving:
> >
> >     Thread A | Thread B | Thread C
> >     ---------+----------+---------
> >        (1)   |          |
> >              |    (1)   |
> >        (2)   |          |
> >        (3)   |          |
> >        (4)   |          |
> >              |    (2)   |
> >              |          |    (1)
> >
> > When this happens, thread B kicks of an unnecessary read. Worse, thread
> > C will see UPTODATE set and return immediately, while the read from
> > thread B is still in progress.  This race could result in tree-checker
> > errors like this as the extent buffer is concurrently modified:
> >
> >     BTRFS critical (device dm-0): corrupted node, root=256
> >     block=8550954455682405139 owner mismatch, have 11858205567642294356
> >     expect [256, 18446744073709551360]
> >
> > Fix it by testing UPTODATE again after setting the READING bit, and if
> > it's been set, skip the unnecessary read.
> >
> > Fixes: d7172f52e993 ("btrfs: use per-buffer locking for extent_buffer reading")
> > Link: https://lore.kernel.org/linux-btrfs/CAHk-=whNdMaN9ntZ47XRKP6DBes2E5w7fi-0U3H2+PS18p+Pzw@mail.gmail.com/
> > Link: https://lore.kernel.org/linux-btrfs/f51a6d5d7432455a6a858d51b49ecac183e0bbc9.1706312914.git.wqu@suse.com/
> > Link: https://lore.kernel.org/linux-btrfs/c7241ea4-fcc6-48d2-98c8-b5ea790d6c89@gmx.com/
> > Signed-off-by: Tavian Barnes <tavianator@tavianator.com>
>
> Thank you very much for taking the time to debug the issue and for the
> fix. It is a rare occurrence that a tough bug is followed by a fix from
> the same person (outside of the developer group) and is certainly
> appreciated.

Thank you!

Sorry to nitpick, but the paragraph you added to the commit message
[1] has a typo:

> There are reports from tree-checker that detects corrupted nodes,
> without any obvious pattern so possibly an overwrite in memory.
> After some debugging it turns out there's a race when reading an extent
> buffer the uptodate status is can be missed.

s/is can/can/

[1]: https://github.com/btrfs/linux/commit/402887e0e9ad76d72496aefebd37bd729748be79

-- 
Tavian Barnes

Re: [PATCH] btrfs: Fix race in read_extent_buffer_pages()

[David Sterba]

On Fri, Mar 22, 2024 at 06:50:07PM -0400, Tavian Barnes wrote:
> On Fri, Mar 22, 2024 at 3:28 PM David Sterba <dsterba@suse.cz> wrote:
> > On Fri, Mar 15, 2024 at 09:14:29PM -0400, Tavian Barnes wrote:
> > > To prevent concurrent reads for the same extent buffer,
> > > read_extent_buffer_pages() performs these checks:
> > >
> > >     /* (1) */
> > >     if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
> > >         return 0;
> > >
> > >     /* (2) */
> > >     if (test_and_set_bit(EXTENT_BUFFER_READING, &eb->bflags))
> > >         goto done;
> > >
> > > At this point, it seems safe to start the actual read operation. Once
> > > that completes, end_bbio_meta_read() does
> > >
> > >     /* (3) */
> > >     set_extent_buffer_uptodate(eb);
> > >
> > >     /* (4) */
> > >     clear_bit(EXTENT_BUFFER_READING, &eb->bflags);
> > >
> > > Normally, this is enough to ensure only one read happens, and all other
> > > callers wait for it to finish before returning.  Unfortunately, there is
> > > a racey interleaving:
> > >
> > >     Thread A | Thread B | Thread C
> > >     ---------+----------+---------
> > >        (1)   |          |
> > >              |    (1)   |
> > >        (2)   |          |
> > >        (3)   |          |
> > >        (4)   |          |
> > >              |    (2)   |
> > >              |          |    (1)
> > >
> > > When this happens, thread B kicks of an unnecessary read. Worse, thread
> > > C will see UPTODATE set and return immediately, while the read from
> > > thread B is still in progress.  This race could result in tree-checker
> > > errors like this as the extent buffer is concurrently modified:
> > >
> > >     BTRFS critical (device dm-0): corrupted node, root=256
> > >     block=8550954455682405139 owner mismatch, have 11858205567642294356
> > >     expect [256, 18446744073709551360]
> > >
> > > Fix it by testing UPTODATE again after setting the READING bit, and if
> > > it's been set, skip the unnecessary read.
> > >
> > > Fixes: d7172f52e993 ("btrfs: use per-buffer locking for extent_buffer reading")
> > > Link: https://lore.kernel.org/linux-btrfs/CAHk-=whNdMaN9ntZ47XRKP6DBes2E5w7fi-0U3H2+PS18p+Pzw@mail.gmail.com/
> > > Link: https://lore.kernel.org/linux-btrfs/f51a6d5d7432455a6a858d51b49ecac183e0bbc9.1706312914.git.wqu@suse.com/
> > > Link: https://lore.kernel.org/linux-btrfs/c7241ea4-fcc6-48d2-98c8-b5ea790d6c89@gmx.com/
> > > Signed-off-by: Tavian Barnes <tavianator@tavianator.com>
> >
> > Thank you very much for taking the time to debug the issue and for the
> > fix. It is a rare occurrence that a tough bug is followed by a fix from
> > the same person (outside of the developer group) and is certainly
> > appreciated.
> 
> Thank you!
> 
> Sorry to nitpick, but the paragraph you added to the commit message
> [1] has a typo:
> 
> > There are reports from tree-checker that detects corrupted nodes,
> > without any obvious pattern so possibly an overwrite in memory.
> > After some debugging it turns out there's a race when reading an extent
> > buffer the uptodate status is can be missed.
> 
> s/is can/can/

I will fix that, thanks.