hujun5
b7c8430de6
Two defects in the CONFIG_TICKET_SPINLOCK paths of spinlock.h:
1. spin_trylock_notrace() passed &lock->owner as the "expected" pointer
of atomic_cmpxchg(). A failed compare-exchange writes the current
value of the target object back through that pointer, so a losing
trylock stores lock->next into lock->owner. owner then equals next,
which is the unlocked state: a lock still held by another CPU reports
itself as free, spin_is_locked() returns false and the lock can be
taken again. Every later unlock keeps incrementing owner past next,
so the ticket of a real waiter never matches and the lock stays
locked forever. Keep the expected value in a local variable.
2. spin_unlock() was wrapped in #ifdef __SP_UNLOCK_FUNCTION, a macro
that is never defined anywhere in the tree. The function body was
therefore dead code and spin_unlock() always expanded to
"do { *(l) = SP_UNLOCKED; } while (0)", which zeroes both ticket
counters instead of releasing one ticket with
atomic_fetch_add(&lock->owner, 1). That drops queued waiters, lets a
newcomer draw ticket 0 and enter the critical section, and also skips
the UP_DMB/UP_DSB/UP_SEV release barriers and the
sched_note_spinlock_unlock() note. Drop the dead #ifdef so
spin_unlock() is always the function.
Both were reproduced on qemu-armv7a:smp (cortex-a7 x4) with
CONFIG_TICKET_SPINLOCK=y, where the compare-exchange lowers to native
ldrex/strex. This confirms the root cause is the C-level aliasing of
the expected pointer, not the atomic implementation.
Refs: https://github.com/apache/nuttx/issues/19808
Signed-off-by: hujun5 <hujun5@xiaomi.com>
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