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Re: [PATCH 1/2] xen/spinlocks: spin_trylock with interrupts off is always fine

On 30.10.20 15:59, Jan Beulich wrote:
On 30.10.2020 15:24, Juergen Gross wrote:
Even if a spinlock was taken with interrupts on before calling
spin_trylock() with interrupts off is fine, as it can't block.

Add a bool parameter "try" to check_lock() for handling this case.

Remove the call of check_lock() from _spin_is_locked(), as it really
serves no purpose and it can even lead to false crashes, e.g. when
a lock was taken correctly with interrupts enabled and the call of
_spin_is_locked() happened with interrupts off. In case the lock is
taken with wrong interrupt flags this will be catched when taking
the lock.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit I guess ...

@@ -42,7 +42,13 @@ static void check_lock(union lock_debug *debug)
       * To guard against this subtle bug we latch the IRQ safety of every
       * spinlock in the system, on first use.
+     *
+     * A spin_trylock() or spin_is_locked() with interrupts off is always
+     * fine, as those can't block and above deadlock scenario doesn't apply.
+    if ( try && irq_safe )
+        return;

... the reference to spin_is_locked() here wants dropping,
since ...

@@ -220,8 +226,6 @@ void _spin_unlock_irqrestore(spinlock_t *lock, unsigned 
long flags)
int _spin_is_locked(spinlock_t *lock)
-    check_lock(&lock->debug);

... you drop the call here?

Oh yes, this was a late modification and I didn't adapt the comment
accordingly. Thanks for spotting it.




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