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authorLinus Torvalds <torvalds@linux-foundation.org>2023-12-08 08:44:43 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2023-12-08 08:44:43 -0800
commit17894c2a7aa60a6da7495cc8500a53523e64c4b1 (patch)
tree4c8c7e0f68c15b2c713a7c4e4369e893db560a5c /MAINTAINERS
parent8e819a7623f19534bce6d53678b581c167b5b079 (diff)
parentf458a1453424e03462b5bb539673c9a3cddda480 (diff)
Merge tag 'trace-v6.7-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace
Pull tracing fixes from Steven Rostedt: - Snapshot buffer issues: 1. When instances started allowing latency tracers, it uses a snapshot buffer (another buffer that is not written to but swapped with the main buffer that is). The snapshot buffer needs to be the same size as the main buffer. But when the snapshot buffers were added to instances, the code to make the snapshot equal to the main buffer still was only doing it for the main buffer and not the instances. 2. Need to stop the current tracer when resizing the buffers. Otherwise there can be a race if the tracer decides to make a snapshot between resizing the main buffer and the snapshot buffer. 3. When a tracer is "stopped" in disables both the main buffer and the snapshot buffer. This needs to be done for instances and not only the main buffer, now that instances also have a snapshot buffer. - Buffered event for filtering issues: When filtering is enabled, because events can be dropped often, it is quicker to copy the event into a temp buffer and write that into the main buffer if it is not filtered or just drop the event if it is, than to write the event into the ring buffer and then try to discard it. This temp buffer is allocated and needs special synchronization to do so. But there were some issues with that: 1. When disabling the filter and freeing the buffer, a call to all CPUs is required to stop each per_cpu usage. But the code called smp_call_function_many() which does not include the current CPU. If the task is migrated to another CPU when it enables the CPUs via smp_call_function_many(), it will not enable the one it is currently on and this causes issues later on. Use on_each_cpu_mask() instead, which includes the current CPU. 2.When the allocation of the buffered event fails, it can give a warning. But the buffered event is just an optimization (it's still OK to write to the ring buffer and free it). Do not WARN in this case. 3.The freeing of the buffer event requires synchronization. First a counter is decremented to zero so that no new uses of it will happen. Then it sets the buffered event to NULL, and finally it frees the buffered event. There's a synchronize_rcu() between the counter decrement and the setting the variable to NULL, but only a smp_wmb() between that and the freeing of the buffer. It is theoretically possible that a user missed seeing the decrement, but will use the buffer after it is free. Another synchronize_rcu() is needed in place of that smp_wmb(). - ring buffer timestamps on 32 bit machines The ring buffer timestamp on 32 bit machines has to break the 64 bit number into multiple values as cmpxchg is required on it, and a 64 bit cmpxchg on 32 bit architectures is very slow. The code use to just use two 32 bit values and make it a 60 bit timestamp where the other 4 bits were used as counters for synchronization. It later came known that the timestamp on 32 bit still need all 64 bits in some cases. So 3 words were created to handle the 64 bits. But issues arised with this: 1. The synchronization logic still only compared the counter with the first two, but not with the third number, so the synchronization could fail unknowingly. 2. A check on discard of an event could race if an event happened between the discard and updating one of the counters. The counter needs to be updated (forcing an absolute timestamp and not to use a delta) before the actual discard happens. * tag 'trace-v6.7-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: ring-buffer: Test last update in 32bit version of __rb_time_read() ring-buffer: Force absolute timestamp on discard of event tracing: Fix a possible race when disabling buffered events tracing: Fix a warning when allocating buffered events fails tracing: Fix incomplete locking when disabling buffered events tracing: Disable snapshot buffer when stopping instance tracers tracing: Stop current tracer when resizing buffer tracing: Always update snapshot buffer size
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