Messages in this thread | | | From | Florian Weimer <> | Subject | Re: Rseq registration: Google tcmalloc vs glibc | Date | Fri, 21 Feb 2020 14:07:58 +0100 |
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Quoting in full to get this message to libc-alpha, past the text/html filter. I wish we had a different list configuration …
Please Note that we have integration patches for glibc which need review. A fair number of them have been written by me, so I can't help with that.
* Chris Kennelly:
> On Thu, Feb 20, 2020 at 4:33 PM Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > wrote: > > Hi Chris, > > As one of the maintainers of the Rseq system call in the Linux kernel, I would > like to thank the Google team for open sourcing a tcmalloc implementation based > on Rseq! > > I've looked into some critical integration aspects of the tcmalloc implementation, > and would like to bring up a topic which involves both tcmalloc developers and the > glibc community. > > Thanks. To answer the later questions first: > * I implemented TCMalloc's upstream rseq based on the conventions I could find from > (mostly) the kernel self tests. This is probably why it looks like #1 :) > > This is less of an intentional preference and more of "it's important that early adopters follow > a convention" for future glibc upgrades. Initializing __rseq_abi is the most important, but > there are some other ones, mostly for debugging/tracing > (https://github.com/compudj/librseq/issues/1), that I'd like to get right too. > > * The TCMalloc project does not provide ABI stability, so TCMalloc can change the convention > it follows. > > I have been discussing aspects of co-existence between early Rseq adopter libraries > and glibc for the past year with the glibc community, and tcmalloc happens to be the > first project to publicly use Rseq outside of prototype branches or selftests code. > Considering that there can only be one Rseq registration per thread (as imposed by > the rseq ABI), there needs to be some kind of protocol between libraries to ensure we > don't introduce regressions when we eventually combine a newer glibc which takes care > of registration of the __rseq_abi TLS along with tcmalloc which also try to perform > that registration within the same thread. > > Throughout the various rounds of review of the glibc Rseq integration patch set, > there were a few solutions envisioned. Here is a brief history: > > 1) Introduce a __rseq_refcount TLS variable. > > - Currently used by Linux tools/testing/selftests/rseq/rseq.c, > - Currently used by Google tcmalloc, > - Emitted by glibc as well my the original patchset (but was later removed), > > A user incrementing the refcount from 0 -> 1 performs rseq registration. > The last user decrementing from 1 -> 0 performs rseq unregistration. > > Works for co-existence of dlopen'd/dlclose'd libraries, for dynamically > linked libraries, and for the main executable. > > The refcounting was deemed too complex for glibc's needs (it always > exists for the entire executable's lifetime), so we moved to > __rseq_handled instead. > > 2) Introduce a __rseq_handled global variable. > > - Currently used by Linux tools/testing/selftests/rseq/rseq.c, > - At some point emitted by glibc as well in my patch set (but was later > removed), > > A library may take rseq ownership if it is still 0 when executing the > library constructor. Set to 1 by library constructor when handling rseq. > Set to 0 in destructor if handling rseq. > > Not meant to be set by dlopen'd/dlclose'd libraries, only by libraries > existing for the whole lifetime of the executable and/or the main executable. > > This __rseq_handled symbol has been identified as being somewhat redundant > with the information provided in the __rseq_abi.cpu_id field (uninitialized > state), which motivated removing this symbol from the glibc integration > entirely. The only reason for having __rseq_handled separate from > __rseq_abi.cpu_id was because it was then impossible to touch TLS data > early in the glibc initialization. This issue was later resolved within > glibc. > > 3) Use the __rseq_abi TLS cpu_id field to know whether Rseq has been > registered. > > - Current protocol in the most recent glibc integration patch set. > - Not supported yet by Linux kernel rseq selftests, > - Not supported yet by tcmalloc, > > Use the per-thread state to figure out whether each thread need to register > Rseq individually. > > Works for integration between a library which exists for the entire lifetime > of the executable (e.g. glibc) and other libraries. However, it does not > allow a set of libraries which are dlopen'd/dlclose'd to co-exist without > having a library like glibc handling the registration present. > > Overall, I like #3 the most due to its simplicity, but I also do not need to support the > dlopen/dlclose use case (below). > > So overall, I suspect the protocol we want for early adopters is that they > only register Rseq if __rseq_abi.cpu_id == RSEQ_CPU_ID_UNINITIALIZED, which > ensure they do not get -1, errno = EBUSY when linked against a newer glibc > which handles Rseq registration. In order to handle multiple early adopters > dlopen'd/dlclose'd in the same executable, those should synchronize with a > __rseq_refcount TLS reference count, but it does not have to be taken into > account by the main executable or libraries present for the entire executable > lifetime (like glibc). > > Based on this, what I think would be missing from the current Google tcmalloc > implementation is a check for __rseq_abi.cpu_id == RSEQ_CPU_ID_UNINITIALIZED > in InitThreadPerCpu(). > > TCMalloc does not get to InitThreadPerCpu without that check. > > Before initialization happens, we > * end up on a slow path > https://github.com/google/tcmalloc/blob/master/tcmalloc/tcmalloc.cc#L1486 > * which checks UsePerCpuCache > https://github.com/google/tcmalloc/blob/master/tcmalloc/cpu_cache.h#L222 > * and inspects the TLS variable in IsFast > https://github.com/google/tcmalloc/blob/master/tcmalloc/internal/percpu.h#L171 > ...which triggers per-thread rseq registration if __rseq_abi.cpu_id is uninitialized. > > Otherwise, the IsOnFastPath() call checks also inspects __rseq_abi.cpu_id via IsFastNoInit > (same thing, but no registration triggered). > > Is tcmalloc ever meant to be dlopen'd/dlclose'd (either directly or indirectly), > or is it required to exist for the entire executable lifetime ? The check and > increment of __rseq_refcount is only useful to co-exist with dlopen'd/dlclose'd > libraries, but it would not allow discovering the presence of a glibc which > takes care of the rseq registration with the planned protocol. A dlopen'd > library should then only perform rseq unregistration if if brings the > __rseq_refcount back to 0 (e.g. in a pthread_key destructor). > > TCMalloc cannot practically be dlopen'd or dlclose'd. > * Once memory is allocated with one instance of malloc (or operator new), it needs to be > free'd to the same heap. > * dlclose is explicitly not supported by our dependencies ("do not rely on dynamic > unloading" https://abseil.io/about/compatibility) > > Thanks, > Chris > > Adding this check for __rseq_abi.cpu_id == RSEQ_CPU_ID_UNINITIALIZED is something > I need to do in the Linux rseq selftests, but I refrained from submitting any > further change to those tests until the glibc rseq integration gets finally > merged. > > Is it something that could be easily changed at this stage in Google tcmalloc, > or should we reconsider adding back __rseq_refcount within the glibc integration > patch set, even though it is not strictly useful to glibc ? > > Thanks, > > Mathieu > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com
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