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bql: Fix bql_locked status with condvar APIs

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QEMU has a per-thread "bql_locked" variable stored in TLS section, showing
whether the current thread is holding the BQL lock.

It's a pretty handy variable.  Function-wise, QEMU have codes trying to
conditionally take bql, relying on the var reflecting the locking status
(e.g. BQL_LOCK_GUARD), or in a GDB debugging session, we could also look at
the variable (in reality, co_tls_bql_locked), to see which thread is
currently holding the bql.

When using that as a debugging facility, sometimes we can observe multiple
threads holding bql at the same time. It's because QEMU's condvar APIs
bypassed the bql_*() API, hence they do not update bql_locked even if they
have released the mutex while waiting.

It can cause confusion if one does "thread apply all p co_tls_bql_locked"
and see multiple threads reporting true.

Fix this by moving the bql status updates into the mutex debug hooks.  Now
the variable should always reflect the reality.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250904223158.1276992-1-peterx@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
This commit is contained in:
Peter Xu 2025-09-04 18:31:58 -04:00 • committed by Philippe Mathieu-Daudé
parent fc2380b568
commit c89d1c879a
4 changed files with 46 additions and 2 deletions
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7
util/qemu-thread-common.h
View file

@ -14,6 +14,7 @@
#define QEMU_THREAD_COMMON_H
#include "qemu/thread.h"
#include "qemu/main-loop.h"
#include "trace.h"
static inline void qemu_mutex_post_init(QemuMutex *mutex)
@ -39,6 +40,9 @@ static inline void qemu_mutex_post_lock(QemuMutex *mutex,
mutex->line = line;
#endif
trace_qemu_mutex_locked(mutex, file, line);
if (mutex_is_bql(mutex)) {
bql_update_status(true);
}
}
static inline void qemu_mutex_pre_unlock(QemuMutex *mutex,
@ -49,6 +53,9 @@ static inline void qemu_mutex_pre_unlock(QemuMutex *mutex,
mutex->line = 0;
#endif
trace_qemu_mutex_unlock(mutex, file, line);
if (mutex_is_bql(mutex)) {
bql_update_status(false);
}
}
#endif