39380e8e01
The span set data structure may leak blocks due to a race in the logic to check whether it's safe to free a block. The simplest example of this race is between two poppers: 1. Popper A claims slot spanSetEntries-2. 2. Popper B claims slot spanSetEntries-1. 3. Popper A gets descheduled before it subtracts from block.used. 4. Popper B subtracts from block.used, sees that claimed spanSetEntries-1, but also that block.used != 0, so it returns. 5. Popper A comes back and subtracts from block.used, but it didn't claim spanSetEntries-1 so it also returns. The spine is left with a stale block pointer and the block later gets overwritten by pushes, never to be re-used again. The problem here is that we designate the claimer of slot spanSetEntries-1 to be the one who frees the block, but that may not be the thread that actually does the last subtraction from block.used. Fixing this problem is tricky, and the fundamental problem there is that block.used is not stable: it may be observed to be zero, but that doesn't necessarily mean you're the last popper! Do something simpler: keep a counter of how many pops have happened to a given block instead of block.used. This counter monotonically increases when a pop is _completely done_. Because this counter is monotonically increasing, and only increases when a popper is done, then we know for sure whichever popper is the last to increase it (i.e. its value is spanSetBlockEntries) is also the last popper in the block. Because the race described above still exists, the last popper may not be the one which claimed the last slot in the block, but we know for certain nobody else is popping from that block anymore so we can safely free it. Finally, because pops serialize with pushes to the same slot, we need not worry about concurrent pushers at all. Updates #37487. Change-Id: I6697219372774c8ca7d8ee6895eaa230a64ce9e1 Reviewed-on: https://go-review.googlesource.com/c/go/+/230497 Run-TryBot: Michael Knyszek <mknyszek@google.com> Reviewed-by: Michael Pratt <mpratt@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> |
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