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SubjectRE: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical
Date
From: Peter Zijlstra
> Sent: 12 November 2019 09:15
...
> > + /*
> > + * If the bitmap is not shared, then nothing can take a refcount as
> > + * current can obviously not fork at the same time. If it's shared
> > + * duplicate it and drop the refcount on the original one.
> > + */
> > + if (refcount_read(&iobm->refcnt) > 1) {
> > + iobm = kmemdup(iobm, sizeof(*iobm), GFP_KERNEL);
> > + if (!iobm)
> > + return -ENOMEM;
> > + io_bitmap_exit();
> refcount_set(&iobm->refcnd, 1);
> > }

What happens if two threads of the same process enter the above
at the same time?
(I've not looked for a lock, but since kmemdup() can sleep I'd suspect there isn't one.)

Also can another thread call fork() - eg while the kmemdup() is sleeping?

David

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