[PATCH v2 0/5] blkdebug: fix racing condition when iterating on

[PATCH v2 0/5] blkdebug: fix racing condition when iterating on

[Emanuele Giuseppe Esposito]

When qemu_coroutine_enter is executed in a loop
(even QEMU_FOREACH_SAFE), the new routine can modify the list,
for example removing an element, causing problem when control
is given back to the caller that continues iterating on the same list. 

Patch 1 solves the issue in blkdebug_debug_resume by restarting
the list walk after every coroutine_enter if list has to be fully iterated.
Patches 2,3,4 aim to fix blkdebug_debug_event by gathering
all actions that the rules make in a counter and invoking 
the respective coroutine_yeld only after processing all requests.

Patch 5 is somewhat independent of the others, it adds a lock to
protect rules and suspended_reqs; right now everything works because
it's protected by the AioContext lock.
This is a preparation for the current proposal of removing the AioContext
lock and instead using smaller granularity locks to allow multiple
iothread execution in the same block device.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
v1 -> v2
* Change commit message of patch 4 and cover letter

Emanuele Giuseppe Esposito (5):
  blkdebug: refactor removal of a suspended request
  blkdebug: move post-resume handling to resume_req_by_tag
  blkdebug: track all actions
  blkdebug: do not suspend in the middle of QLIST_FOREACH_SAFE
  blkdebug: protect rules and suspended_reqs with a lock

 block/blkdebug.c | 113 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 79 insertions(+), 34 deletions(-)

-- 
2.30.2

[PATCH v2 1/5] blkdebug: refactor removal of a suspended request

[Emanuele Giuseppe Esposito]

Extract to a separate function.  Do not rely on FOREACH_SAFE, which is
only "safe" if the *current* node is removed---not if another node is
removed.  Instead, just walk the entire list from the beginning when
asked to resume all suspended requests with a given tag.

Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block/blkdebug.c | 28 +++++++++++++++++-----------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/block/blkdebug.c b/block/blkdebug.c
index 2c0b9b0ee8..8f19d991fa 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -793,7 +793,6 @@ static void suspend_request(BlockDriverState *bs, BlkdebugRule *rule)
         printf("blkdebug: Resuming request '%s'\n", r.tag);
     }
 
-    QLIST_REMOVE(&r, next);
     g_free(r.tag);
 }
 
@@ -869,25 +868,35 @@ static int blkdebug_debug_breakpoint(BlockDriverState *bs, const char *event,
     return 0;
 }
 
-static int blkdebug_debug_resume(BlockDriverState *bs, const char *tag)
+static int resume_req_by_tag(BDRVBlkdebugState *s, const char *tag, bool all)
 {
-    BDRVBlkdebugState *s = bs->opaque;
-    BlkdebugSuspendedReq *r, *next;
+    BlkdebugSuspendedReq *r;
 
-    QLIST_FOREACH_SAFE(r, &s->suspended_reqs, next, next) {
+retry:
+    QLIST_FOREACH(r, &s->suspended_reqs, next) {
         if (!strcmp(r->tag, tag)) {
+            QLIST_REMOVE(r, next);
             qemu_coroutine_enter(r->co);
+            if (all) {
+                goto retry;
+            }
             return 0;
         }
     }
     return -ENOENT;
 }
 
+static int blkdebug_debug_resume(BlockDriverState *bs, const char *tag)
+{
+    BDRVBlkdebugState *s = bs->opaque;
+
+    return resume_req_by_tag(s, tag, false);
+}
+
 static int blkdebug_debug_remove_breakpoint(BlockDriverState *bs,
                                             const char *tag)
 {
     BDRVBlkdebugState *s = bs->opaque;
-    BlkdebugSuspendedReq *r, *r_next;
     BlkdebugRule *rule, *next;
     int i, ret = -ENOENT;
 
@@ -900,11 +909,8 @@ static int blkdebug_debug_remove_breakpoint(BlockDriverState *bs,
             }
         }
     }
-    QLIST_FOREACH_SAFE(r, &s->suspended_reqs, next, r_next) {
-        if (!strcmp(r->tag, tag)) {
-            qemu_coroutine_enter(r->co);
-            ret = 0;
-        }
+    if (resume_req_by_tag(s, tag, true) == 0) {
+        ret = 0;
     }
     return ret;
 }
-- 
2.30.2

[PATCH v2 2/5] blkdebug: move post-resume handling to resume_req_by_tag

[Emanuele Giuseppe Esposito]

We want to move qemu_coroutine_yield() after the loop on rules,
because QLIST_FOREACH_SAFE is wrong if the rule list is modified
while the coroutine has yielded.  Therefore move the suspended
request to the heap and clean it up from the remove side.
All that is left is for blkdebug_debug_event to handle the
yielding.

Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block/blkdebug.c | 31 ++++++++++++++++++-------------
 1 file changed, 18 insertions(+), 13 deletions(-)

diff --git a/block/blkdebug.c b/block/blkdebug.c
index 8f19d991fa..e37f999254 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -775,25 +775,20 @@ static void blkdebug_close(BlockDriverState *bs)
 static void suspend_request(BlockDriverState *bs, BlkdebugRule *rule)
 {
     BDRVBlkdebugState *s = bs->opaque;
-    BlkdebugSuspendedReq r;
+    BlkdebugSuspendedReq *r;
 
-    r = (BlkdebugSuspendedReq) {
-        .co         = qemu_coroutine_self(),
-        .tag        = g_strdup(rule->options.suspend.tag),
-    };
+    r = g_new(BlkdebugSuspendedReq, 1);
+
+    r->co         = qemu_coroutine_self();
+    r->tag        = g_strdup(rule->options.suspend.tag);
 
     remove_rule(rule);
-    QLIST_INSERT_HEAD(&s->suspended_reqs, &r, next);
+    QLIST_INSERT_HEAD(&s->suspended_reqs, r, next);
 
     if (!qtest_enabled()) {
-        printf("blkdebug: Suspended request '%s'\n", r.tag);
+        printf("blkdebug: Suspended request '%s'\n", r->tag);
     }
     qemu_coroutine_yield();
-    if (!qtest_enabled()) {
-        printf("blkdebug: Resuming request '%s'\n", r.tag);
-    }
-
-    g_free(r.tag);
 }
 
 static bool process_rule(BlockDriverState *bs, struct BlkdebugRule *rule,
@@ -875,8 +870,18 @@ static int resume_req_by_tag(BDRVBlkdebugState *s, const char *tag, bool all)
 retry:
     QLIST_FOREACH(r, &s->suspended_reqs, next) {
         if (!strcmp(r->tag, tag)) {
+            Coroutine *co = r->co;
+
+            if (!qtest_enabled()) {
+                printf("blkdebug: Resuming request '%s'\n", r->tag);
+            }
+
             QLIST_REMOVE(r, next);
-            qemu_coroutine_enter(r->co);
+            g_free(r->tag);
+            g_free(r);
+
+            qemu_coroutine_enter(co);
+
             if (all) {
                 goto retry;
             }
-- 
2.30.2

[PATCH v2 3/5] blkdebug: track all actions

[Emanuele Giuseppe Esposito]

Add a counter for each action that a rule can trigger.
This is mainly used to keep track of how many coroutine_yeld()
we need to perform after processing all rules in the list.

Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block/blkdebug.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/block/blkdebug.c b/block/blkdebug.c
index e37f999254..388b5ed615 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -74,6 +74,7 @@ enum {
     ACTION_INJECT_ERROR,
     ACTION_SET_STATE,
     ACTION_SUSPEND,
+    ACTION__MAX,
 };
 
 typedef struct BlkdebugRule {
@@ -791,22 +792,22 @@ static void suspend_request(BlockDriverState *bs, BlkdebugRule *rule)
     qemu_coroutine_yield();
 }
 
-static bool process_rule(BlockDriverState *bs, struct BlkdebugRule *rule,
-    bool injected)
+static void process_rule(BlockDriverState *bs, struct BlkdebugRule *rule,
+                         int *action_count)
 {
     BDRVBlkdebugState *s = bs->opaque;
 
     /* Only process rules for the current state */
     if (rule->state && rule->state != s->state) {
-        return injected;
+        return;
     }
 
     /* Take the action */
+    action_count[rule->action]++;
     switch (rule->action) {
     case ACTION_INJECT_ERROR:
-        if (!injected) {
+        if (action_count[ACTION_INJECT_ERROR] == 1) {
             QSIMPLEQ_INIT(&s->active_rules);
-            injected = true;
         }
         QSIMPLEQ_INSERT_HEAD(&s->active_rules, rule, active_next);
         break;
@@ -819,21 +820,19 @@ static bool process_rule(BlockDriverState *bs, struct BlkdebugRule *rule,
         suspend_request(bs, rule);
         break;
     }
-    return injected;
 }
 
 static void blkdebug_debug_event(BlockDriverState *bs, BlkdebugEvent event)
 {
     BDRVBlkdebugState *s = bs->opaque;
     struct BlkdebugRule *rule, *next;
-    bool injected;
+    int actions_count[ACTION__MAX] = { 0 };
 
     assert((int)event >= 0 && event < BLKDBG__MAX);
 
-    injected = false;
     s->new_state = s->state;
     QLIST_FOREACH_SAFE(rule, &s->rules[event], next, next) {
-        injected = process_rule(bs, rule, injected);
+        process_rule(bs, rule, actions_count);
     }
     s->state = s->new_state;
 }
-- 
2.30.2

[PATCH v2 4/5] blkdebug: do not suspend in the middle of QLIST_FOREACH_SAFE

[Emanuele Giuseppe Esposito]

That would be unsafe in case a rule other than the current one
is removed while the coroutine has yielded.
Keep FOREACH_SAFE because suspend_request deletes the current rule.

After this patch, *all* matching rules are deleted before suspending
the coroutine, rather than just one.
This doesn't affect the existing testcases.

Use actions_count to see how many yeld to issue.

Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block/blkdebug.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/block/blkdebug.c b/block/blkdebug.c
index 388b5ed615..dffd869b32 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -789,7 +789,6 @@ static void suspend_request(BlockDriverState *bs, BlkdebugRule *rule)
     if (!qtest_enabled()) {
         printf("blkdebug: Suspended request '%s'\n", r->tag);
     }
-    qemu_coroutine_yield();
 }
 
 static void process_rule(BlockDriverState *bs, struct BlkdebugRule *rule,
@@ -834,6 +833,12 @@ static void blkdebug_debug_event(BlockDriverState *bs, BlkdebugEvent event)
     QLIST_FOREACH_SAFE(rule, &s->rules[event], next, next) {
         process_rule(bs, rule, actions_count);
     }
+
+    while (actions_count[ACTION_SUSPEND] > 0) {
+        qemu_coroutine_yield();
+        actions_count[ACTION_SUSPEND]--;
+    }
+
     s->state = s->new_state;
 }
 
-- 
2.30.2

[PATCH v2 5/5] blkdebug: protect rules and suspended_reqs with a lock

[Emanuele Giuseppe Esposito]

Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block/blkdebug.c | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/block/blkdebug.c b/block/blkdebug.c
index dffd869b32..e92a35ccbb 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -54,6 +54,7 @@ typedef struct BDRVBlkdebugState {
     /* For blkdebug_refresh_filename() */
     char *config_file;
 
+    QemuMutex lock;
     QLIST_HEAD(, BlkdebugRule) rules[BLKDBG__MAX];
     QSIMPLEQ_HEAD(, BlkdebugRule) active_rules;
     QLIST_HEAD(, BlkdebugSuspendedReq) suspended_reqs;
@@ -245,7 +246,9 @@ static int add_rule(void *opaque, QemuOpts *opts, Error **errp)
     };
 
     /* Add the rule */
+    qemu_mutex_lock(&s->lock);
     QLIST_INSERT_HEAD(&s->rules[event], rule, next);
+    qemu_mutex_unlock(&s->lock);
 
     return 0;
 }
@@ -468,6 +471,7 @@ static int blkdebug_open(BlockDriverState *bs, QDict *options, int flags,
     int ret;
     uint64_t align;
 
+    qemu_mutex_init(&s->lock);
     opts = qemu_opts_create(&runtime_opts, NULL, 0, &error_abort);
     if (!qemu_opts_absorb_qdict(opts, options, errp)) {
         ret = -EINVAL;
@@ -568,6 +572,7 @@ static int blkdebug_open(BlockDriverState *bs, QDict *options, int flags,
     ret = 0;
 out:
     if (ret < 0) {
+        qemu_mutex_destroy(&s->lock);
         g_free(s->config_file);
     }
     qemu_opts_del(opts);
@@ -582,6 +587,7 @@ static int rule_check(BlockDriverState *bs, uint64_t offset, uint64_t bytes,
     int error;
     bool immediately;
 
+    qemu_mutex_lock(&s->lock);
     QSIMPLEQ_FOREACH(rule, &s->active_rules, active_next) {
         uint64_t inject_offset = rule->options.inject.offset;
 
@@ -595,6 +601,7 @@ static int rule_check(BlockDriverState *bs, uint64_t offset, uint64_t bytes,
     }
 
     if (!rule || !rule->options.inject.error) {
+        qemu_mutex_unlock(&s->lock);
         return 0;
     }
 
@@ -606,6 +613,7 @@ static int rule_check(BlockDriverState *bs, uint64_t offset, uint64_t bytes,
         remove_rule(rule);
     }
 
+    qemu_mutex_unlock(&s->lock);
     if (!immediately) {
         aio_co_schedule(qemu_get_current_aio_context(), qemu_coroutine_self());
         qemu_coroutine_yield();
@@ -771,8 +779,10 @@ static void blkdebug_close(BlockDriverState *bs)
     }
 
     g_free(s->config_file);
+    qemu_mutex_destroy(&s->lock);
 }
 
+/* Called with lock held.  */
 static void suspend_request(BlockDriverState *bs, BlkdebugRule *rule)
 {
     BDRVBlkdebugState *s = bs->opaque;
@@ -791,6 +801,7 @@ static void suspend_request(BlockDriverState *bs, BlkdebugRule *rule)
     }
 }
 
+/* Called with lock held.  */
 static void process_rule(BlockDriverState *bs, struct BlkdebugRule *rule,
                          int *action_count)
 {
@@ -829,17 +840,22 @@ static void blkdebug_debug_event(BlockDriverState *bs, BlkdebugEvent event)
 
     assert((int)event >= 0 && event < BLKDBG__MAX);
 
+    qemu_mutex_lock(&s->lock);
     s->new_state = s->state;
     QLIST_FOREACH_SAFE(rule, &s->rules[event], next, next) {
         process_rule(bs, rule, actions_count);
     }
 
+    qemu_mutex_unlock(&s->lock);
+
     while (actions_count[ACTION_SUSPEND] > 0) {
         qemu_coroutine_yield();
         actions_count[ACTION_SUSPEND]--;
     }
 
+    qemu_mutex_lock(&s->lock);
     s->state = s->new_state;
+    qemu_mutex_unlock(&s->lock);
 }
 
 static int blkdebug_debug_breakpoint(BlockDriverState *bs, const char *event,
@@ -862,11 +878,14 @@ static int blkdebug_debug_breakpoint(BlockDriverState *bs, const char *event,
         .options.suspend.tag = g_strdup(tag),
     };
 
+    qemu_mutex_lock(&s->lock);
     QLIST_INSERT_HEAD(&s->rules[blkdebug_event], rule, next);
+    qemu_mutex_unlock(&s->lock);
 
     return 0;
 }
 
+/* Called with lock held.  */
 static int resume_req_by_tag(BDRVBlkdebugState *s, const char *tag, bool all)
 {
     BlkdebugSuspendedReq *r;
@@ -884,7 +903,9 @@ retry:
             g_free(r->tag);
             g_free(r);
 
+            qemu_mutex_unlock(&s->lock);
             qemu_coroutine_enter(co);
+            qemu_mutex_lock(&s->lock);
 
             if (all) {
                 goto retry;
@@ -898,8 +919,12 @@ retry:
 static int blkdebug_debug_resume(BlockDriverState *bs, const char *tag)
 {
     BDRVBlkdebugState *s = bs->opaque;
+    int rc;
 
-    return resume_req_by_tag(s, tag, false);
+    qemu_mutex_lock(&s->lock);
+    rc = resume_req_by_tag(s, tag, false);
+    qemu_mutex_unlock(&s->lock);
+    return rc;
 }
 
 static int blkdebug_debug_remove_breakpoint(BlockDriverState *bs,
@@ -909,6 +934,7 @@ static int blkdebug_debug_remove_breakpoint(BlockDriverState *bs,
     BlkdebugRule *rule, *next;
     int i, ret = -ENOENT;
 
+    qemu_mutex_lock(&s->lock);
     for (i = 0; i < BLKDBG__MAX; i++) {
         QLIST_FOREACH_SAFE(rule, &s->rules[i], next, next) {
             if (rule->action == ACTION_SUSPEND &&
@@ -921,6 +947,7 @@ static int blkdebug_debug_remove_breakpoint(BlockDriverState *bs,
     if (resume_req_by_tag(s, tag, true) == 0) {
         ret = 0;
     }
+    qemu_mutex_unlock(&s->lock);
     return ret;
 }
 
@@ -929,11 +956,14 @@ static bool blkdebug_debug_is_suspended(BlockDriverState *bs, const char *tag)
     BDRVBlkdebugState *s = bs->opaque;
     BlkdebugSuspendedReq *r;
 
+    qemu_mutex_lock(&s->lock);
     QLIST_FOREACH(r, &s->suspended_reqs, next) {
         if (!strcmp(r->tag, tag)) {
+            qemu_mutex_unlock(&s->lock);
             return true;
         }
     }
+    qemu_mutex_unlock(&s->lock);
     return false;
 }
 
-- 
2.30.2

Re: [PATCH v2 3/5] blkdebug: track all actions

[Eric Blake]

On 5/7/21 10:12 AM, Emanuele Giuseppe Esposito wrote:
> Add a counter for each action that a rule can trigger.
> This is mainly used to keep track of how many coroutine_yeld()

yield

> we need to perform after processing all rules in the list.
> 
> Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
>  block/blkdebug.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v2 4/5] blkdebug: do not suspend in the middle of QLIST_FOREACH_SAFE

[Eric Blake]

On 5/7/21 10:12 AM, Emanuele Giuseppe Esposito wrote:
> That would be unsafe in case a rule other than the current one
> is removed while the coroutine has yielded.
> Keep FOREACH_SAFE because suspend_request deletes the current rule.
> 
> After this patch, *all* matching rules are deleted before suspending
> the coroutine, rather than just one.
> This doesn't affect the existing testcases.
> 
> Use actions_count to see how many yeld to issue.

yield

> 
> Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
>  block/blkdebug.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v2 5/5] blkdebug: protect rules and suspended_reqs with a lock

[Eric Blake]

On 5/7/21 10:12 AM, Emanuele Giuseppe Esposito wrote:
> Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
>  block/blkdebug.c | 32 +++++++++++++++++++++++++++++++-
>  1 file changed, 31 insertions(+), 1 deletion(-)
> 

> @@ -929,11 +956,14 @@ static bool blkdebug_debug_is_suspended(BlockDriverState *bs, const char *tag)
>      BDRVBlkdebugState *s = bs->opaque;
>      BlkdebugSuspendedReq *r;
>  
> +    qemu_mutex_lock(&s->lock);
>      QLIST_FOREACH(r, &s->suspended_reqs, next) {
>          if (!strcmp(r->tag, tag)) {
> +            qemu_mutex_unlock(&s->lock);
>              return true;
>          }
>      }
> +    qemu_mutex_unlock(&s->lock);
>      return false;

Would code like this be easier to write by using QEMU_LOCK_GUARD from
lockable.h?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v2 5/5] blkdebug: protect rules and suspended_reqs with a lock

[Paolo Bonzini]

On 07/05/21 17:29, Eric Blake wrote:
>> +    qemu_mutex_lock(&s->lock);
>>       QLIST_FOREACH(r, &s->suspended_reqs, next) {
>>           if (!strcmp(r->tag, tag)) {
>> +            qemu_mutex_unlock(&s->lock);
>>               return true;
>>           }
>>       }
>> +    qemu_mutex_unlock(&s->lock);
>>       return false;
> Would code like this be easier to write by using QEMU_LOCK_GUARD from
> lockable.h?

Yes, this one would.  In other cases (rule_check) it's not so clear cut. 
  It depends whether you prefer to have the simplest code, or rather to 
have homogeneous use of either guards or lock/unlock.

Paolo

Re: [PATCH v2 5/5] blkdebug: protect rules and suspended_reqs with a lock

[Emanuele Giuseppe Esposito]

On 11/05/2021 10:37, Paolo Bonzini wrote:
> On 07/05/21 17:29, Eric Blake wrote:
>>> +    qemu_mutex_lock(&s->lock);
>>>       QLIST_FOREACH(r, &s->suspended_reqs, next) {
>>>           if (!strcmp(r->tag, tag)) {
>>> +            qemu_mutex_unlock(&s->lock);
>>>               return true;
>>>           }
>>>       }
>>> +    qemu_mutex_unlock(&s->lock);
>>>       return false;
>> Would code like this be easier to write by using QEMU_LOCK_GUARD from
>> lockable.h?
> 
> Yes, this one would.  In other cases (rule_check) it's not so clear cut. 
>   It depends whether you prefer to have the simplest code, or rather to 
> have homogeneous use of either guards or lock/unlock.

Makes sense. I will use the lock guard and fix the "yield" typos in the 
other patches.

Thank you,
Emanuele