Date: Fri, 11 Jan 2013 17:48:53 -0600 From: Christian Peron <csjp@sqrt.ca> To: Guy Helmer <guy.helmer@gmail.com> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org> Subject: Re: bpf hold buffer in-use flag Message-ID: <0AAF6A81-2594-449E-A38D-28DE5B055AFF@sqrt.ca> In-Reply-To: <9C928117-2230-4F01-9B95-B6D945AF4416@gmail.com> References: <9C928117-2230-4F01-9B95-B6D945AF4416@gmail.com>
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Guy,
Can you please describe the race and how to reproduce it? When we =
introduced zerocopy-bpf, we also introduced bpf_bufheld() and =
bpf_bufreclaimed() which are effectively hops for the regular buffer =
mode. Maybe we could maintain state there as well? In any case, I would =
like to understand the race/and reproduce it
Thanks!
On 2012-11-13, at 3:40 PM, Guy Helmer wrote:
> To try to completely resolve the race in bpfread(), I have put =
together these changes to add a flag to indicate when the hold buffer =
cannot be modified because it is in use. Since it's my first time using =
mtx_sleep() and wakeup(), I wanted to run these past the list to see if =
I can get any feedback on the approach.
>=20
>=20
> Index: bpf.c
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> --- bpf.c (revision 242997)
> +++ bpf.c (working copy)
> @@ -819,6 +819,7 @@ bpfopen(struct cdev *dev, int flags, int fmt, stru
> * particular buffer method.
> */
> bpf_buffer_init(d);
> + d->bd_hbuf_in_use =3D 0;
> d->bd_bufmode =3D BPF_BUFMODE_BUFFER;
> d->bd_sig =3D SIGIO;
> d->bd_direction =3D BPF_D_INOUT;
> @@ -872,6 +873,9 @@ bpfread(struct cdev *dev, struct uio *uio, int iof
> callout_stop(&d->bd_callout);
> timed_out =3D (d->bd_state =3D=3D BPF_TIMED_OUT);
> d->bd_state =3D BPF_IDLE;
> + while (d->bd_hbuf_in_use)
> + mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
> + PRINET|PCATCH, "bd_hbuf", 0);
> /*
> * If the hold buffer is empty, then do a timed sleep, which
> * ends when the timeout expires or when enough packets
> @@ -940,27 +944,27 @@ bpfread(struct cdev *dev, struct uio *uio, int =
iof
> /*
> * At this point, we know we have something in the hold slot.
> */
> + d->bd_hbuf_in_use =3D 1;
> BPFD_UNLOCK(d);
>=20
> /*
> * Move data from hold buffer into user space.
> * We know the entire buffer is transferred since
> * we checked above that the read buffer is bpf_bufsize bytes.
> - *
> - * XXXRW: More synchronization needed here: what if a second =
thread
> - * issues a read on the same fd at the same time? Don't want =
this
> - * getting invalidated.
> + *
> + * We do not have to worry about simultaneous reads because
> + * we waited for sole access to the hold buffer above.
> */
> error =3D bpf_uiomove(d, d->bd_hbuf, d->bd_hlen, uio);
>=20
> BPFD_LOCK(d);
> - if (d->bd_hbuf !=3D NULL) {
> - /* Free the hold buffer only if it is still valid. */
> - d->bd_fbuf =3D d->bd_hbuf;
> - d->bd_hbuf =3D NULL;
> - d->bd_hlen =3D 0;
> - bpf_buf_reclaimed(d);
> - }
> + KASSERT(d->bd_hbuf !=3D NULL, ("bpfread: lost bd_hbuf"));
> + d->bd_fbuf =3D d->bd_hbuf;
> + d->bd_hbuf =3D NULL;
> + d->bd_hlen =3D 0;
> + bpf_buf_reclaimed(d);
> + d->bd_hbuf_in_use =3D 0;
> + wakeup(&d->bd_hbuf_in_use);
> BPFD_UNLOCK(d);
>=20
> return (error);
> @@ -1114,6 +1118,9 @@ reset_d(struct bpf_d *d)
>=20
> BPFD_LOCK_ASSERT(d);
>=20
> + while (d->bd_hbuf_in_use)
> + mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock, PRINET,
> + "bd_hbuf", 0);
> if ((d->bd_hbuf !=3D NULL) &&
> (d->bd_bufmode !=3D BPF_BUFMODE_ZBUF || bpf_canfreebuf(d))) =
{
> /* Free the hold buffer. */
> @@ -1254,6 +1261,9 @@ bpfioctl(struct cdev *dev, u_long cmd, caddr_t =
add
>=20
> BPFD_LOCK(d);
> n =3D d->bd_slen;
> + while (d->bd_hbuf_in_use)
> + mtx_sleep(&d->bd_hbuf_in_use, =
&d->bd_lock,
> + PRINET, "bd_hbuf", 0);
> if (d->bd_hbuf)
> n +=3D d->bd_hlen;
> BPFD_UNLOCK(d);
> @@ -1967,6 +1977,9 @@ filt_bpfread(struct knote *kn, long hint)
> ready =3D bpf_ready(d);
> if (ready) {
> kn->kn_data =3D d->bd_slen;
> + while (d->bd_hbuf_in_use)
> + mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
> + PRINET, "bd_hbuf", 0);
> if (d->bd_hbuf)
> kn->kn_data +=3D d->bd_hlen;
> } else if (d->bd_rtout > 0 && d->bd_state =3D=3D BPF_IDLE) {
> @@ -2299,6 +2312,9 @@ catchpacket(struct bpf_d *d, u_char *pkt, u_int =
pk
> * spot to do it.
> */
> if (d->bd_fbuf =3D=3D NULL && bpf_canfreebuf(d)) {
> + while (d->bd_hbuf_in_use)
> + mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
> + PRINET, "bd_hbuf", 0);
> d->bd_fbuf =3D d->bd_hbuf;
> d->bd_hbuf =3D NULL;
> d->bd_hlen =3D 0;
> @@ -2341,6 +2357,9 @@ catchpacket(struct bpf_d *d, u_char *pkt, u_int =
pk
> ++d->bd_dcount;
> return;
> }
> + while (d->bd_hbuf_in_use)
> + mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
> + PRINET, "bd_hbuf", 0);
> ROTATE_BUFFERS(d);
> do_wakeup =3D 1;
> curlen =3D 0;
> Index: bpf.h
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> --- bpf.h (revision 242997)
> +++ bpf.h (working copy)
> @@ -1235,7 +1235,8 @@ SYSCTL_DECL(_net_bpf);
> /*
> * Rotate the packet buffers in descriptor d. Move the store buffer =
into the
> * hold slot, and the free buffer ino the store slot. Zero the length =
of the
> - * new store buffer. Descriptor lock should be held.
> + * new store buffer. Descriptor lock should be held. Hold buffer =
must
> + * not be marked "in use".
> */
> #define ROTATE_BUFFERS(d) do { =
\
> (d)->bd_hbuf =3D (d)->bd_sbuf; =
\
> Index: bpf_buffer.c
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> --- bpf_buffer.c (revision 242997)
> +++ bpf_buffer.c (working copy)
> @@ -189,6 +189,9 @@ bpf_buffer_ioctl_sblen(struct bpf_d *d, u_int *i)
> return (EINVAL);
> }
>=20
> + while (d->bd_hbuf_in_use)
> + mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
> + PRINET, "bd_hbuf", 0);
> /* Free old buffers if set */
> if (d->bd_fbuf !=3D NULL)
> free(d->bd_fbuf, M_BPF);
> Index: bpfdesc.h
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> --- bpfdesc.h (revision 242997)
> +++ bpfdesc.h (working copy)
> @@ -63,6 +63,7 @@ struct bpf_d {
> caddr_t bd_sbuf; /* store slot */
> caddr_t bd_hbuf; /* hold slot */
> caddr_t bd_fbuf; /* free slot */
> + int bd_hbuf_in_use; /* don't rotate buffers */
> int bd_slen; /* current length of store =
buffer */
> int bd_hlen; /* current length of hold buffer =
*/
>=20
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