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MINOR: fd: add a lock bit with the tgid

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In order to permit to migrate FDs from one thread group to another,
we'll need to be able to set a TGID that is compatible with no other
thread group. Either we use a special value or we dedicate a special
bit. Given that we already have way more bits than needed, let's just
sacrifice the topmost one to serve as a lock bit, indicating the tgid
is not valid anymore. This will make all fd_grab_tgid() fail to grab
it.

The new fd_lock_tgid() function now tries to assign a locked tgid to
an idle FD, and fd_unlock_tgid() simply drops the lock bit, revealing
the target tgid.

For now it's still unused so it must not have any effect.
This commit is contained in:
Willy Tarreau 2023-04-13 14:27:48 +02:00
parent 4d882bd800
commit 7b44c26e13
2 changed files with 38 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
include/haproxy/fd-t.h
View File

@ -179,9 +179,10 @@ struct fdlist {
* 32-bit small archs can put everything in 32-bytes when threads are disabled.
* refc_tgid is an atomic 32-bit composite value made of 16 higher bits
* containing a refcount on tgid and the running_mask, and 16 lower bits
* containing a thread group ID. The tgid may only be changed when refc is zero
* and running may only be checked/changed when refc is held and shows the
* reader is alone. An FD with tgid zero belongs to nobody.
* containing a thread group ID and a lock bit on the 16th. The tgid may only
* be changed when refc is zero and running may only be checked/changed when
* refc is held and shows the reader is alone. An FD with tgid zero belongs to
* nobody.
*/
struct fdtab {
unsigned long running_mask; /* mask of thread IDs currently using the fd */

35
include/haproxy/fd.h
View File

@ -330,6 +330,39 @@ static forceinline void fd_drop_tgid(int fd)
HA_ATOMIC_SUB(&fdtab[fd].refc_tgid, 0x10000);
}
/* Unlock a tgid currently locked by fd_lock_tgid(). This will effectively
* allow threads from the FD's tgid to check the masks and manipulate the FD.
*/
static forceinline void fd_unlock_tgid(int fd)
{
HA_ATOMIC_AND(&fdtab[fd].refc_tgid, 0xffff7fffU);
}
/* Switch the FD's TGID to the new value with a refcount of 1 and the lock bit
* set. It doesn't care about the current TGID, except that it will wait for
* the FD not to be already switching and having its refcount cleared. After
* the function returns, the caller is free to manipulate the masks, and it
* must call fd_unlock_tgid() to drop the lock, allowing threads from the
* designated group to use the FD. Finally a call to fd_drop_tgid() will be
* needed to drop the reference.
*/
static inline void fd_lock_tgid(int fd, uint desired_tgid)
{
uint old;
BUG_ON(!desired_tgid);
old = tgid; // assume we start from the caller's tgid
desired_tgid |= 0x18000; // refcount=1, lock bit=1.
while (1) {
old &= 0x7fff; // expect no lock and refcount==0
if (_HA_ATOMIC_CAS(&fdtab[fd].refc_tgid, &old, desired_tgid))
break;
__ha_cpu_relax();
}
}
/* Grab a reference to the FD's TGID, and return the tgid. Note that a TGID of
* zero indicates the FD was closed, thus also fails (i.e. no need to drop it).
* On non-zero (success), the caller must release it using fd_drop_tgid().
@ -387,7 +420,7 @@ static inline void fd_claim_tgid(int fd, uint desired_tgid)
if (_HA_ATOMIC_CAS(&fdtab[fd].refc_tgid, &old, desired_tgid))
break;
__ha_cpu_relax();
old &= 0xffff;
old &= 0x7fff; // keep only the tgid and drop the lock
}
}