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Re: [PATCH] nptl: Implement pthread_self in libc.so
On 12/14/2017 09:50 PM, Florian Weimer wrote:
> On 12/15/2017 05:51 AM, Carlos O'Donell wrote:
>> On 12/14/2017 10:37 AM, Florian Weimer wrote:
>>> All binaries use TLS and thus need a properly set up TCB, so we can
>>> simply return its address directly, instead of forwarding to the
>>> libpthread implementation from libc.
>> Could you expand on this a bit?
>> Yes, all binaries use TLS by virtue of errno.
>> Yes, because we need errno setup we need a TCB setup.
>> However, internally we never call pthread_self, we use THREAD_SELF,
>> and access the thread register directly. So there is no internal cost
>> paid for pthread_self.
> I don't understand this. The existing implementation has a cost
> because of the redirect from libc.so to libpthread.so:
> Dump of assembler code for function pthread_self:
> <+0>: mov 0x2b505a(%rip),%eax # <__libc_pthread_functions_init>
> <+6>: test %eax,%eax
> <+8>: je 0x7ffff76f0690 <pthread_self+32>
> <+10>: mov 0x2b4fb7(%rip),%rax # <__libc_pthread_functions+280>
> <+17>: ror $0x11,%rax
> <+21>: xor %fs:0x30,%rax
> <+30>: jmpq *%rax
> <+32>: xor %eax,%eax
> <+34>: retq
> Compare this to the implementation in libpthread (which is what the
> new libc.so implementation looks like as well):
> Dump of assembler code for function __pthread_self:
> <+0>: mov %fs:0x10,%rax
> <+9>: retq
> The redirect implementation returns zero as a pthread_t value, which
> is not correct if libpthread is loaded later (something this redirect
> functionality is there to support, but which is currently defective
> in various ways).
Let me clarify.
Your change has no performance implications for libc.so, since we never
call pthread_self, we always use THREAD_SELF, the macro.
Your change *does* have a performance implication for applications calling
pthread_self without having linked against libpthread.so.
This is, as you say, an optimization.
>> The only cost paid is if the application binary calls pthread_self to
>> use that descriptor in some way, and to use it in some way means calling
>> a pthread function with it.
> It's also common to use it for logging, although that is a slight
> abuse. The range of permitted uses is certainly limited because
> pthread_t is an unsigned long, and any direct manipulation is
> The stub implementation is broken, so we can't move more
> functionality from libpthread to libc until it is fixed anyway.
That could be fixed for just this case?
>> Are there thread functions we might use in a program not linked against
>> libpthread e.g. the pthread_atfork changes we have in Fedora?
> pthread_thread_number_np depends on this change in practical terms.
Why? In pthread_thread_number_np you can call THREAD_SELF directly?
>>> The layout of struct pthread_functions is not changed by this commit,
>>> although the pointer used to store the address of pthread_self is now
> To preserve the internal ABI. It seem to have very little cost. I
> can remove the field, of course.
I would prefer the removal. Please submit a v2.
>>> To avoid linking against the libpthread function in all cases, we would
>>> have to bump the symbol version of libpthread in libc.so and supply a
>>> compat symbol.
>> Could you expand on what this might look like?
> We'd add pthread_self@@GLIBC_2.27 to glibc and turn the existing
> version into a compat symbol. Both would be backed by the same
> implementation (at the same address). Hopefully, this will encourage
> link editors not to use the compatibility symbol in libpthread—it
> then has a completely different version, so it should be treated as
I agree this is a little excessive when we already have the forwarders
in place, and can keep dlopen(pthread_self) working.
>>> This commit does not do that because the function
>>> implementation is so small, so the overhead by two active copies of the
>>> same function might well be smaller than the increase in symbol table
>> Can we please measure that to be sure?
> I don't see how. Is it really necessary? The new implementation in
> libc is clearly an improvement.
I think I see your point better now that you explained it in more detail.
The solution you propose is simpler.
> The link editor should probably be fixed not to link against
> compatibility symbols in certain cases.
Well, I think we should remove the symbol<->DSO error that prevents us
from migrating versioned symbols across DSO boundaries. This should be