Re: [PATCH] mm: Make failslab writable again

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On 20.09.22 12:29, Vlastimil Babka wrote:
On 9/20/22 11:17, Alexander Atanasov wrote:
Hello,

On 20.09.22 11:42, Vlastimil Babka wrote:
+static ssize_t failslab_store(struct kmem_cache *s, const char *buf,
+                size_t length)
+{
+    if (s->refcount > 1)
+        return -EINVAL;
+
+    s->flags &= ~SLAB_FAILSLAB;
+    if (buf[0] == '1')
+        s->flags |= SLAB_FAILSLAB;

Could we at least use a temporary variable to set up the final value and
then do a WRITE_ONCE() to s->flags, so the compiler is not allowed to do
some funky stuff? Assuming this is really the only place where we modify
s->flags during runtime, so we can't miss other updates due to RMW.

Since it is set or clear - instead of temporary variable and potentially two
writes and RMW issues i would suggest this:
+    if (buf[0] == '1')
+        s->flags |= SLAB_FAILSLAB;
+       else
+        s->flags &= ~SLAB_FAILSLAB;

This way also has RMW issues, and also the compiler is allowed to
temporarily modify s->flags any way it likes; with WRITE_ONCE() it can't.

Okay, so the safest way is this?

if (buf[0] == '1')
	WRITE_ONCE(s->flags, READ_ONCE(s->flags) | SLAB_FAILSLAB);
else
	WRITE_ONCE(s->flags, READ_ONCE(s->flags) & ~SLAB_FAILSLAB);

It got me thinking how many places would break if the compiler
starts to temporariliy modify the flags - i hope it never does.

--
Regards,
Alexander Atanasov





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