Messages in this thread | | | From | David Laight <> | Subject | RE: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical | Date | Thu, 14 Nov 2019 13:22:03 +0000 |
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From: Peter Zijlstra <peterz@infradead.org> > On Thu, Nov 14, 2019 at 11:02:01AM +0000, David Laight wrote: > > 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? > > Suppose there's just the two threads, and both will change it. Then both > do copy-on-write and the original gets freed.
I was probably forgetting that the linux kernel uses (more or less) full 'process' structures for threads, rather than separate 'thread' data areas.
David
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