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i965/batch: Ensure we use a consistent offset in relocs 

In theory gcc is free to re-load them, and if a concurrent
execbuf races and updates bo->offset64 then we have a problem:
execbuffer api requires that the ->presumed_offset and the one
we used for the reloc matches. It does not require that the value
is sensible, which means no locks needed, just a consistent load.

Ken said his next series will nuke this, so just hand-roll the
kernel's READ_ONCE idea inline.

FIXME: Most callers of brw_emit_reloc recompute the relocation
themselves, which means this doesn't really fix the race. But the long
term plan is to move to per-context relocation handling, which will
fix this all properly. So leave this for now as just a reminder.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
This commit is contained in:
Daniel Vetter 2017-04-06 09:13:47 +02:00 committed by Kenneth Graunke
parent 7f3c85c21e
commit a99a4979fd
1 changed files with 6 additions and 2 deletions
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8
src/mesa/drivers/dri/i965/intel_batchbuffer.c
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@ -733,6 +733,8 @@ brw_emit_reloc(struct intel_batchbuffer *batch, uint32_t batch_offset,
struct brw_bo *target, uint32_t target_offset,
uint32_t read_domains, uint32_t write_domain)
{
uint64_t offset64;
if (batch->reloc_count == batch->reloc_array_size) {
batch->reloc_array_size *= 2;
batch->relocs = realloc(batch->relocs,
@ -752,18 +754,20 @@ brw_emit_reloc(struct intel_batchbuffer *batch, uint32_t batch_offset,
batch->reloc_count++;
/* ensure gcc doesn't reload */
offset64 = *((volatile uint64_t *)&target->offset64);
reloc->offset = batch_offset;
reloc->delta = target_offset;
reloc->target_handle = target->gem_handle;
reloc->read_domains = read_domains;
reloc->write_domain = write_domain;
reloc->presumed_offset = target->offset64;
reloc->presumed_offset = offset64;
/* Using the old buffer offset, write in what the right data would be, in
* case the buffer doesn't move and we can short-circuit the relocation
* processing in the kernel
*/
return target->offset64 + target_offset;
return offset64 + target_offset;
}
void