Messages in this thread |  | | Date | Wed, 12 Jun 2002 18:38:31 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] 2.5.21 Nonlinear CPU support |
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Anton Altaparmakov wrote: > > The code would be run outside the critical region... But correct about > the race. I thought that was obvious and wasn't suggesting the above to be > the actual code... That was supposed to be obvious from lack of error > handling etc... Never mind. My mistake, I should have been more precise > the first time round, here is the actual code I had in mind: > > [snip] > if (unlikely(!ntfs_compression_buffers)) { > int err; > > /* > * This code path only ever triggers once so we take it > * out of line. > */ > if ((err = try_to_allocate_compression_buffers())) { > // TODO: do appropriate cleanups > return err; > } > } > disable_preempt(); > cb = ntfs_compression_buffers[smp_processor_id()]; > [snip] > > and try_to_allocate_compression_buffers would be: > > int try_to_allocate_compression_buffers(void) > { > int err = 0; > > down(&ntfs_lock); > if (likely(!ntfs_compression_buffers)) > err = allocate_compression_buffers(); > up(&ntfs_lock); > return err; > } > > and allocate_compression_buffers() is the same as it is now. Actually I > was going to fuse try_to_allocate and allocate into one function but as I > am showing above it is clearer to see what I had in mind... > > Happy now? This basically just defers the allocation to a bit later. As it > is at the moment the allocation happens at mount time of a partition which > supports compression. Note that the code in super.c would still need to > exist due to reference counting so we know when we can free the buffers > again. The only thing changed in super.c will be to remove the actual call > to allocate_compression_buffers, all else stays in place. Otherwise we > have no way to tell when we can throw away the buffers. >
I presume allocate_compression_buffers() allocates *all* buffers, and doesn't return error if there is nothing to allocate? If so, the above code should be OK.
If allocate_compression_buffers() either doesn't check if it has already allocated, or returns an error if buffers were already allocated, then the above code is OK *EXCEPT IN THE CASE OF HOTSWAP CPUs*.
My originally proposed code allocated one buffer at a time, and should be correct even in the presence of hotswap CPUs.
-hpa
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