[PATCH] gdb: don't use inferior_ptid in linux_nat_wait_1

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[PATCH] gdb: don't use inferior_ptid in linux_nat_wait_1

Sourceware - gdb-patches mailing list
From: Simon Marchi <[hidden email]>

target_ops::wait implementations should not rely on the value of
inferior_ptid on entry.  While looking at another wait-related patch, I
noticed that the code in linux_nat_wait_1, checking for a newly created
process, did just that.  This patch fixes it.  Note that I didn't see
any bug, this "fix" is simply to make the function respect the
target_ops::wait contract.

Instead of checking inferior_ptid, check for the passed in `ptid`
value.

During startup, linux_nat_wait_1 gets called a few times with the
pid-only ptid, while startup_inferior waits for the expected number of
exec events.  For this reason, I needed to add a `find_lwp_pid` call to
ensure that the actions of changing the main thread's ptid, and adding
the initial lwp, were done only once for a given process.

This was not needed before, since thread_change_ptid, through the
thread_ptid_changed observer, ends up changing inferior_ptid.  So the
second time around, inferior_ptid was not a pid-only ptid.

That find_lwp_pid won't add much overhead, as it will only be called
when the ptid is a pid-only ptid.  And AFAIK, that only happens during
inferior startup.

An alternative to that `find_lwp_pid` call might be to make
startup_inferior realize that the main thread has changed ptid, and make
it wait for the new ptid.  But that doesn't look easy to do.

Regtested on amd64/Linux.

gdb/ChangeLog:

        * linux-nat.c (linux_nat_wait_1): Don't use inferior_ptid when
        checking for initial lwp.

Change-Id: I8f1d5c766f5cb2a29c948bc75fa4582d7130c23f
---
 gdb/linux-nat.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index 20b03bc2ba9..75c6d219d6a 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -3277,14 +3277,13 @@ linux_nat_wait_1 (ptid_t ptid, struct target_waitstatus *ourstatus,
   /* The first time we get here after starting a new inferior, we may
      not have added it to the LWP list yet - this is the earliest
      moment at which we know its PID.  */
-  if (inferior_ptid.is_pid ())
+  if (ptid.is_pid () && find_lwp_pid (ptid) == nullptr)
     {
-      /* Upgrade the main thread's ptid.  */
-      thread_change_ptid (linux_target, inferior_ptid,
-  ptid_t (inferior_ptid.pid (),
-  inferior_ptid.pid (), 0));
+      ptid_t lwp_ptid (ptid.pid (), ptid.pid ());
 
-      lp = add_initial_lwp (inferior_ptid);
+      /* Upgrade the main thread's ptid.  */
+      thread_change_ptid (linux_target, ptid, lwp_ptid);
+      lp = add_initial_lwp (lwp_ptid);
       lp->resumed = 1;
     }
 
--
2.28.0

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Re: [PATCH] gdb: don't use inferior_ptid in linux_nat_wait_1

Tom Tromey-2
Simon> gdb/ChangeLog:

Simon> * linux-nat.c (linux_nat_wait_1): Don't use inferior_ptid when
Simon>         checking for initial lwp.

This seems fine to me.
Thanks.

Tom
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Re: [PATCH] gdb: don't use inferior_ptid in linux_nat_wait_1

Simon Marchi-4
On 2020-08-04 2:04 p.m., Tom Tromey wrote:
> Simon> gdb/ChangeLog:
>
> Simon> * linux-nat.c (linux_nat_wait_1): Don't use inferior_ptid when
> Simon>         checking for initial lwp.
>
> This seems fine to me.
> Thanks.

Thanks for checking.  I'll wait to see if Pedro has an opinion about this, just
in case.

Simon
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Re: [PATCH] gdb: don't use inferior_ptid in linux_nat_wait_1

Simon Marchi-4
On 2020-08-04 2:49 p.m., Simon Marchi wrote:

> On 2020-08-04 2:04 p.m., Tom Tromey wrote:
>> Simon> gdb/ChangeLog:
>>
>> Simon> * linux-nat.c (linux_nat_wait_1): Don't use inferior_ptid when
>> Simon>         checking for initial lwp.
>>
>> This seems fine to me.
>> Thanks.
>
> Thanks for checking.  I'll wait to see if Pedro has an opinion about this, just
> in case.
>
> Simon
>

I pushed this patch.

Simon